So you'd like to construct your own hamster cage? There several ways to construct one, depending on whether you want your cage to be made "from scratch" or converted from a pre-existing container.
Basically, there are two requirements for a hamster cage: to keep the hamsters from escaping, and to let the owner (you) watch what they're doing. A solid wood or metal box with a lid on it would meet the first purpose well, but you'd need to open the lid to see what your pets are doing. A better solution is to have one or more sides of the cage made of some transparent material: a solid piece of glass or plastic, or a wire mesh.
The easiest, and the most economical, way to construct a hamster cage from another container is to convert a see-through plastic storage. The minimum size for the common Syrian hamster is 24 inches by 12 inches in floor space. But bigger is always better -- use the best one you can find or can afford.
The minimum furnishings for a hamster cage are bedding material and a water source. The most common bedding material is wood shavings which you can buy at a pet store. With a plastic container, you'll probably need to use a water bottle that has a freestanding stand, since it may be difficult to attach a water bottle to the sides of some plastic containers. Another option is to use Velcro to attach the water bottle to the side of the container.
If you've got an old aquarium sitting around it will work well as a hamster cage too. (Aquariums can be made of glass or of plastic; plastic is lighter, but scratches more easily.) DO NOT put a clear aquarium hamster cage near a windowsill that gets a lot of sun, because the cage may act as a heat trap.
WIRE MESH CAGES
There aren't many conversions that can be made here: if it wasn't made as a cage, it probably can't be used as a cage. If you have some skills and experience with building things, though, building your own wire mesh cage is an option. Go to your local public library, used bookstore, or poke around on the internet, and look for hobby and craft books that were originally published over 40 years ago. Many of these books will have detailed instructions for constructing inexpensive, sturdy and attractive hamster cages from various materials.