Foam board (Blue works best from my understanding as it is aquarium safe) Carve the design you are looking for and then cover in drylok (acrylic version) and add sand for texture if desired. You can tint the dry lock to color it. Or use expanding foam and carve it. Grout would also work to cover either one but you will need to let it set and do many water changes to adjust the ph or cover the grout in drylok. Do an internet search for DYI aquarium backgrounds and you'll find lots of great ideas. Good luck!