Aimee, I mentioned earlier our building flats from scratch for a big set. Actually, we had loads of older flats accumulated over the years, but they were heavy, mismatched, etc. etc. So we bit the bullet and made a whole set of new flats. Maybe you should do the same thing. Tear apart the old flats and use the lumber for other things. When you decide what the height of your set (flats) will be , say 10 or 12 feet, you may build flats of that height and shorter flats that go over doors and windows. So, each door, for example, will be framed by three flats. One on each side the height of the set, and one on top connecting the two side flats to complete the "wall." Ordinarily, you don't want your doors connected to the flats. Doors should be free-standing, independent of the flats, supported by their own system of braces etc. so that when the actors slam the doors the vibation isn't transmitted to the flats making them flap and shake. Or, forget all I said and get a good book on set building.

It is Muslin. TO build a flat you need to strech out the muslin really tight and then use something we call shrinky dink. It is a surprising simple soulution of mainly water and gule I think it is 50/50. Does that help??