Check out the above thread. I made this planter box to grow food. If you made the exact same thing, but only put 3-4" of dirt in it, instead of filling it to the top as I did, it makes a perfect tortoise table. It is very easy to build too. Only two cuts with a circular saw on the 2x12 end pieces, and simply screw it all together.
For substrate, my two favorites are orchid bark and coco coir.
I would suggest whatever you do end up making, make it a double tiered enclsoure. You can add a lot more room for the tortoises, without taking up a lot more floor space in your home.