There are two kinds of hardware cloth, galvanized and plastic coated. Galvanized wire is cheaper but it is also less comfortable and can absorb odors after a long period of use. Plastic coated wire looks nicer and is more comfortable for both the rats and you, but it is a bit more expensive and when the rats chew the wire the plastic strips away, leaving bits of silver wire showing. Plastic coated wire is found almost exclusively at Home Depot and comes in a 2' x 5' roll. Make sure you get the 1/2" bar spacing variety and not the larger size which is too wide for rats. For this particular cage you will need two rolls, but if your cage is smaller or larger you will need less or more.
Plastic Storage Bin ($7.97 for the one used in this cage)
The cheapest and easiest way to make a bottom pan for your cage is to buy a plastic storage tote. They come in many different sizes, but the ones most suitable for cage building are the under-bed storage variety as they are low but are still large. Select your bin based on how large you want the cage to be, keeping in mind that if you can't find one large enough you can always make your cage taller to add room. All of the measurements for the rest of the cage depend on the size of the tote used, so it is a good idea to buy this first before you get any wire.
Cable ties, also known as zip-ties, come in very handy for building rat cages as they are strong, sturdy, and resist chewing but they can be removed easily if you need to. If you would like a more permanent method, use J-clips instead of cable ties.
J-Clips & J-Clip Pliers
J-clips are small metal bands shaped like a letter J and form a metal ring around two pieces of wire, attaching them together. You will need a J-clip plier to properly attach them. They are impossible to chew and create a sturdy, strong hold, but they are very difficult to remove if need be. Even if you use cable ties instead, it is a good idea to use J-clips for the door hinges.
Door Latches
There are many ways to fasten the doors of your rat cage shut, and how you wish to do that is up to you. There are commercially made latches which can be bought online, or you can make one yourself out of a spring, a ring, and an S-hook. You can also use a short bungy cord or twist ties.
Tools
To put the cage together you will need pliers, a measuring tape, and wire cutters (often included in the pliers). Optional tools include tin snips or a large kitchen knife.
Cut the Wire
Measure the length and width of the inside of your plastic bottom to find out how long each wire section needs to be. The cage can be as tall as you like. Unroll the wire carefully and flatten it out as best you can. It doesn't have to be perfect just yet. Measure and cut the four sides of the cage, the two shorter sides and the longer front and back. If you have enough wire you can make a top as well, but in this particular example there was not enough wire and an alternative method was used for the lid. When cutting the wire, leave the ends long on each piece so you can fold the long pieces back to create a smooth edge (see picture below). Wire can be very sharp and can hurt your rat as well as you while you are building or moving the cage if you don't bend the ends over in this fashion. Another option would be to use plastic strips along the edges but they will most likely be chewed to pieces.
Once the edges have been folded over, it is safer to straighten the pieces out so they lay flat. The straighter they are the easier your cage will fit together.
Using either cable ties or J-clips, set the sides inside the plastic pan to make sure they fit and then attach them together. You'll want to space the fasteners about 5 squares apart at minimum to ensure the rats cannot push between them. If you are making a wire top as well, do not attach it yet or it will be more difficult to put in the levels.
Choose the straightest side for the front, as the doors will close more easily if the front panel is flat. You can cut the doors now if you wish, but it would be better to wait until the levels are finished so you know exactly where to make them.
Once you have the sides fastened together it's time to add your levels. Use any scrap pieces of wire if you can, to save the larger pieces for the doors or ceiling. It is up to you how many levels you want and how large you want them to be. Making a ramp to each level is also a good idea. Cable tie or J-clip the levels on to the sides so they are secure. If you think you may change your mind about the levels before the cage is done, use twist ties to temporarily hold them in place until you have decided for certain.
Now that you know where your levels are going to be, you can decide where you want the doors. Keep in mind that you will have to be able to reach into every corner of the cage easily and the doors will have to be large enough so you can easily remove a rat. You will also have to decide whether you want the doors to open to the left, right, top, or bottom. When you have the size chosen, cut out the holes, keeping the edges long so you can fold them over as shown in the picture to the left.
Cut the doors so they are about two squares wider than the hole on every side, then use either cable ties or J-clips to fasten the door. J-clips work a bit better and they also function well as a hinge. You can now attach your door latch. Make sure the door closes securely and the rats cannot squeeze their way out. In this example twist ties were used as a temporary closure, but bungy cords were purchased for a latch later on.
If you made your top out of wire you can simply attach it with J-clips or cable ties. If you don't have enough wire, you can use the lid to the storage bin you used for the bottom pan. You can either use cable ties to fasten it permanently or several longer bungy cords if you want to be able to remove the lid for easier access. If you have some scrap wire left over, you can also cut a hole in the lid and fasten the wire to it for some added ventilation, as shown in the picture to the left.
To help escape proof your cage, as well as make it easier to carry, you may want to use bungy cords or a spring and S-hook as a fastener to affix the wire section to the plastic pan. It should be something that can be removed easily when it comes time to clean the cage but is strong enough to support the weight of the bottom when the cage is lifted from the top.
Add Accessories
Now's the time to add the finishing touches. A food dish, water bottle, hammocks, tunnels, a wheel, and many other things can be added to make your cage a great place for your rat to live. The more surface area the better!