I do something similar to what Johnny described. I use PVC of half to 1 inch diameter. You can cut this to the height of the tank. IF you buy plastic clamps that fit inside the pipe you can use the clamps to hold the pipes where you want them in the tank. Use bigger pipe and bigger clamps for bigger fish. I sand down, rough up, one side of the pipe and use silicon (aquarium selant) glue to glue plastic canvas to the side of the pipe. I just cut it to fit the tank prior to gluing it to the pipes.
THe PVC and clamps are available at Lowes. The clamps are also more readily available at Sears. And the plastic canvas at WallMart and other places that sell cloth.