I wouldn't use cement, so many poblems with wate chemistry. i would just get some foam, carve it to shape, and apply a few layes of epoxy to it. as far as cycling, I would use the screens on the intake caves to block the fishes, and just clean them ev ery water change. I would also add the fishes you have after 24 hours of setup, and then add more stock gradually. make sure you let the epoxy cure for 24 hrs or so, and then fill it, let it soak for 24 hs, do a complete water change, fill, let sit for 24 hrs and then start adding stock.