Your head location is similar to that on a Triton. When I first got my boat there was a glassed plywood
holding tank below the port settee, but it ate up to much of my limited storage space so we moved it to the area behind the head, again a custom glassed plywood construction. We did investigate various ready made tanks, but could not find one that suited the Triton, however, your A30 has just enough extra room that one of these tanks may work for you.