I am sure there would be. I don't have any plans, but I am sure with a little research on kingsnake.com, we can find something. It might be easier to buy something, but if you are determined to build something, I am sure we can go that route.
I have one concern though, homemade cages are notorious for having holes the snakes use to escape. I have had three snakes escape from "escape proof" cages built for snakes. (I read the owner's manual to them and explained that kind of behavior wasn't acceptable....)
I would also point out that garters are cheap, but not good snakes. They eat a lot of different things (that's good), but they tend to smell more and they will musk you. (That will be bad.) Garters don't eat mice, so you'll have some trouble getting them appropriate food. I will admit they could be fun, but I'd steer clear of them. I suggest you get a book on them, before you go that direction. In fact, I think I have one I can loan you if you are interested.
I'd suggest a corn snake. You can get them pretty cheap in a couple of months on line, or at a local reptile show. The are the best pets and are a mouse's worst nightmare. Once they start eating, you usually won't have any problems with them.