What substrate do you use for the flooring of your runs? I have a 8' wide by 16' long area I want to use for my run. I was going to use concrete blocks for the border, and put them down either 1-1 1/2 blocks deep, this will be my foundation for the walls. I was then going to place 1/2-1inch of pea gravel for drainage, then a few inches of sand on top. Will this be ok?