The wood is untreated pine available in various widths from hardware shops. This is then made into tunnels using a saw, a drill, sandpaper, screws and a screwdriver - joining blocks are also useful. The lid is made from wire mesh cut to size and stapled into untreated pine surrounds. Toys can be incorporated into the design. I have a wooden ladder made for birds in one of mine and some plastic guttering and cardboard poster tubes in another.
Gerbils enjoy running around it - lots of scope for exercise.
Great fun to design and build if you are interested in that sort of thing. Tricky to clean - must be dismantled with a screwdriver. They tend to use the wood as a toilet so the wood has to be desinfected as well. About 1-2 days to build, less if you get proficient at it. The principle is simple, like a normal fish tank, only the glass is less thick.