I have to say, in all my years of pro photography with lots of sets built, I've never seen flats made of canvas or muslin. Having a theatre background myself, and having built more than my share in the method JT describes, I still wouldn't use those. Generally, we use a very thin wooden material which is called "luan" (I don't know how to spell it, its pronounced "loo-ann" of you ask at a lumber yard.) Its basically a 1/4" sheet of lightweight wood. You still attach it to the 1x3 frame JT mentioned, but its just more like a real wall when you paint or texture it. And you don't really need it 10' tall unless you're shooting basketball players. Foamcore would work too, but it gets a curl in it when painted, and would still need to be attached to frame to keep it flat (unless you get 1/2" thick, in which case you might not need a frame).
Regards,
Andy Pearlman
Andy Pearlman Studio