Plastic canvas! The hole's are just right to weave hemp, jute, vines etc through it to create a solid but lightweight second level platform.
I'm going to be making one for my tank and suspending it from the lid. I'm also going to make a railing out of vines so my hermies don't fall off by accident.

If a person got really creative they could use different shades (if it's safe) to make designs in the floor.