HV,
Are you matching the existing tank? Making mounting pads for a new tank is pretty simple. Just match where your mounting areas are. You find a material say1" thk., glass it to the hull & glass over it. Use some ss screws with caulk.
If I were you, first I'd call my friend Lenny @ Catalina in largo Florida. They make all kinds of stuff for the C30's. (727) 544-6681
If you do go this route, I would also have them put in an inspection cover on top. Go with a min. of 4" either square or round. This will allow you to clean any sludge without having to pay someone to do it. Catalina have good pricing on remaking orig. parts.
CR