Stainless steel flat head wood screws. Pre dill each hole, counter sink and put a screw in it for a dry install. Careful not the spin it, you just need it tight enough to ensure it fits right..
Remove the screws, cross hatch sand both surfaces 80 grit, clean with acetone, put thicken epoxy where glass touches glass (no need to put epoxy on the foam). Dip the screws threads in epoxy to screw them in for the final install.
The filler between the sponson and the hull should be cabosil. Talc, microspheres, cotton flock, milled fibers and actual glass can all be substituted if necessary, but cabosil is best.
Cut off excess screws on the inside after it has cured. Put filler (microspheres or talc is best) over them if desired.
Use can use cabosil on the bottom. It will hold up well, but it will be a bear to sand so don't glob it on.
Talc as a filler is much easier to sand for finishing work. Microspheres leave pin holes and are not great for final finishing for paint. For gelcoat you can use microspheres any place I suggested talc.
3:1 laminating epoxy. If you already ordered your supplies and want to use talc, the baby powder at the dollar store will work just as well as composite talc, it's just that the fragrances will cause a color, probably greenish, not an issue if you gelcoat. I am a big fan of talc these days and buy it with my other composites. Microspheres leave holes and everything else sucks to sand.
This is where my experience ends. The paint/gelcoat I am not well versed in.
Realize that gelcoat is poly based and you are using epoxy resin. You may have some varied results. I have minimal experience with gelcoat, but poly resin layups over epoxy/SMC don't last. I also have very limited paint experience and can't advise you gelcoat vs epoxy. Just advising you to make sure you do your research on the topic since it could be an issue but I don't know enough about it to advise you properly.
