I would have steel mesh on the roof so that it's open to the elements.
My two like to run about in the rain - it would be really loud with a perspex roof on, and too dark with roofing felt.
We bought the steel mesh from B&Q - it doesn't rust and the cats can climb the sides like spiderman. You can get the cat friendly wood from any woodyard - it isn't coated with anything. The staff there will know which wood to get.
I'm after one of those excercise wheels too but haven't found any in the UK.

Ali x

On the subject of wheels - I'm suppose to be making some when I get the time!

Elaine (Wildside) was suppose to be organising some kind of race between me and Steve if I remember right....I get the feeling Steve didn't know he was signed up for wheel making duties!

Anyhoo I am planning to make a start on one soon.....I think the only way to get one ATM is to import really, and that is expensive trust me!

Unless you get someone who's handy to make one up for you....again this wil be very pricey.