Hi! I am building an outside pen for my redfoot pair. I live in South Florida, so during the summer, they should do well outside. Unfortunately, my husband won't give up his lawn service, so keeping them on the grass will not be possible. I have to build a pen, on a concrete slab. ( tortoises won't be touching the slab) Of course, I will need a large amount of substrate , in deep layers, in the pen I am building with concrete walls, and a cover for protection. Any ideas of what the best choice would be for the substrate? I want to use some soil, so I can plant in the pen. Any input/ ideas for the enclosure would be much appreciated!