Building a Loft

I just got a new cage for the hedgehog I'll be getting in April, and have been looking over various posts about cage setup, and I LOVE the idea of a loft! I don't have a ton of horizontal space in my house right now (I'm moving in May, and will have more room then, and already have plans for horizontal cage expansion), but I could definitely add a loft into the cage I already have!

I'm not entirely sure how to go about building one, though. I know that it needs to be closed on the sides so he won't fall, and that a tube is the best ramp, but what materials should I use to build it, and, once it's built, how can I attach it in the cage? I'm thinking of using C&C stuff to make the loft (with plastic covering the bottom). Would that work? I'm going to start searching, but does anyone know what Canadian stores carry that kind of stuff?

My cage is 40.5 inches long, 18 inches wide and 20.5 inches high, with horizontal bars attached with a vertical bar every few inches around. http://www.petsmart.com/product/index.jsp?productId=2753290 I'll obviously measure everything first to make sure it'll fit in a manner that won't make my hedgie bump his little head. If anyone has some suggestions, I'd really appreciate it!