@Swell_Watercraft, this may be a little aside of the topic, but while you’re here maybe you could help me understand something I have a vague theory about.
It goes like this: All the rotomolded PE SOTs I’ve seen have some variety of linear folding in their hulls. Not the case with at least one thermoformed boat, the Hurricane Skimmer. My thinking is that the PE needs those folds to maintain its shape and not act like a water balloon. I’ll grant that the thermoformed boats may have foam blocks in them for structural stiffness–Bic, at least, says so straight up about their Scapa, but they also use PE in a tri-hull shape.
I believe you’re on to something with the material. Evidently–foam blocks or no–ABS is a lot lighter than PE; witness the 50-pound Skimmer and Caribbean, compared to Bic’s Java, 13’5" x 28" and 64 lbs, even as a two-sheet construction rather than rotomolded. They do not mention the use of foam in that boat.
Now, I’m not sure what this “2-layer foam backed plastic” of yours is, but I can envision a 3-layer material with fairly rigid outsides sandwiching a rigid but mostly hollow honeycomb plastic. In the book “Structures”, J.E. Gordon talks about the importance of well-engineered holes. Of course this would require sheet-formed parts, but it may be something to look into, if you haven’t already.
If it takes 18 pounds off the weight of the boat, it’ll float higher and accumulate less water in the footwells.
Finally, can you show us some pictures of the bottom of your boat?