In the last year I have discovered how useful styrofoam and plaster of paris have been in my cage designs. Right now I'm building 4 4'by2'by2' cages with plywood and all have a background with built in hides and basking spots all home made. Making hides easily and cheaply is only a three step process. I personally cut several pieces of 1 inch think styrofoam and simply glue them together with a foamboard glue in one of those cylinders. Once I about have the shape I want I use a hot knife that cuts styrofoam very easily to make it less angular and more natural by cutting down on the corners. Lastly I coat the thing with a couple times with plaster of paris mixed with cement dye giving it a nice grey rockish color. This is actually a very easy process that is way more cost efficient than buying 10 dollar hides from the pet store. Also it gives you the opportunity to make the hide any way you want, big , small, low, or even fitting into a corner. Sorry about the long post but this process has worked very well for me and my pets.
-Erik