First off, I think your idea of making a cage for your rats is a great one, and you'd be surprised how easy it can be to do.....and the supplies to do it are good and cheap!
Here is a website where I think the woman has come up with a really great, and easy plan for making huge rat cages:
Hopefully this will give you some ideas and help you come up with a plan. Another idea is to keep an eye out in the paper for folks selling various old cages for cheap. Ferret cages, bird cages....many things can be picked up cheap, and converted into fantastic rattie havens. You can even combine and hook together more than one cage.
Good luck, I hope you find something, I'm sure you can, and your rats will thank you for it!
