Just about any white material that will diffuse the light will work. Most stuff like this is made from one of the white nylons types. Some use rip stop nylon, but I've perferred to use something called sport nylon. It's a bit thicker material and doesn't have the little cross lines (the rip stop threads) running through it. But I have also used polyester type material (I think I found it in the wedding gown section of the fabric store) as well as white (bleached) muslin and good ole white bed sheets cut up.
Really, it doesn't matter that much what kind of material it is as long as it's white (you still will probably have to do a custom white balance with it) and thick enough to diffuse the light and spread it around over the tent. You don't even need to make a square tent either. Some of the ones being sold today are like a big tee pee (cone shapped with the pointed up pointed up), with the material drapped down over the object.
Mike