Interesting question. I suppose if I wanted something a bit more for the outboard screws I might opt for a strip of stainless perhaps 1/8th thick by 5/8ths wide and say 5 or 6 inches long drilled with clearance holes much like a narrow backing plate that could be easily inserted into the hull and liner gap. Then a few nuts to secure and there would be some additional strength. Another option might be to pot the T nuts in an inverted position (long end down) with the flange against the deck and encease them in thickened epoxy from the inside, again utilizing the gap for access. Kind of messy though and tough to work on considering the narrow gap. The biggest issue we encounter is the end of the core envelope as it stops about an inch from the hull. With this there is a very pronounced step. Another option would be to build up the step to the envelope thickness using cloth and epoxy, again messy, and redrill the outboard holes for a T nut to pull up into. Just some alternate ideas beyond the nut and washer.