So I ordered a a Moeller FT-5606-R1. A Christmas gift to myself.
I got it sort of local.
http://www.oceanlinkinc.com is the 2nd closest place to me that carries it, and the first didn't have a retail store. These people do.
I hope I measured correctly. marine liquidators in FLA had one that was close, and might have fit in the aft half of the compartment but it was too close to tell. Hatch cover flex might have compromised a fitting, and that would have been really bad.
I have a plan for molding that conforming tank support so that I can let it rest on the stringers and fully support the tank bottom and sides, allowing for some expansion.