Would love to build something like this! If you are planning it to be mobile (I'm assuming so with the wheels), how are you predator proofing around the bottom to allow it to roll?
If you end up going the bagged sand route, Home Depot offered broken bags to me for half off. I've asked since then and they've given them to me again discounted. Just bring tape with you!
Love the colors, the L shape, and also the use of the space under the nesting box for run space! Had not thought of that... It just hangs out anyway, might as well use it for more run.