I'd say that BlueWotsit had it pretty much right at the start of this thread, as long as the hull is made from polyester resin and not epoxy, all you need is resin mixed with some filler, P38 is good, Plastic Padding very similar, and I use the one sold under their own brand by motor factors Brown Brothers. You add enough filler to the resin to get whatever consistency you want for the job in hand and the amount of gap-filling that's needed. As with any job of this kind, the most important single factor is surface preparation, if you don't do a good job here, you're wasting your time whatever material you use. I know that there are other fillers, glass-filled, microballoons etc, but I've never used any of them. With good surface preparation and ordinary filler or a filler/resin mix, I can get a good enough bond with a propshaft/hull joint to break the fibreglass before the joint gives way, what more could anyone need? There's no need for any complication here, just keep it simple.