There is some pretty big PVC
available out there. I have seen 4 inch stuff at the big box stores. You could put several shapes together, including a couple "T"s to give you openings. And you can use a jig saw or hack saw to cut chunks out, too. (Be sure to clamp it down before cutting.) Glue the pieces together with tank-safe silicone, and then paint it with Krylon Fusion. Use a color that will blend with whatever you are going to glue on top. I used a dark brown on PVC
, and then used tank-safe silicone to glue brown gravel over the paint. That way, any missed spots do not show.
Just be sure to let the Krylon Fusion dry for at least a week before you start gluing stuff to it or put it under water.