Well, I found a solution to one of the interior finish issues - gluing the Reflectix insulation to the
fiberglass shell.
None of the spray adhesives performed very well - 3M Super 77, Lepages Heavy Duty or LocTite. I have 6 cans of 3M Hi Strength 90 on order, but they are going to take weeks to get here and I can't delay the rest of the project that long. So I found an alternative.
TiteBond Fast Grab FRP Adhesive - it's used for gluing up FRP panels. It goes on with a notched trowel like Thinset or Mastic, rather than spraying and you only have to glue one of the surfaces, not both. It doesn't seem to matter which surface. Since gluing the back of the ReFlectix is a lot easier than working overhead with a trowel, that's what I did.
The initial grab is fast, but you have several minutes of working time to reposition - depending on how long you take to apply it. But once it's set, it is way stronger than contact adhesive. On a test piece that I tore apart, the Reflectix failed before the glue joint. It has been warm here in Vancouver - 25C - 30C (75F - 85F) and it seems to stand up to heat pretty well, but it has not been as not (40C / 105F) as late June when the 3M Super 77 failed. However I don't expect it to fail in heat because unlike contact adhesive, it actually hardens and doesn't remain plastic.
It dries to an opaque "greige" colour. For this reason and because I don't know if it will bleed through fabric, I will probably use the 3M 90 on the headliner fabric.