If there are any planes you want air tight but dont want to fiberglass, theres a pretty easy way to do it. Cut a wooden trim panel (usually out of hardboard or some really thin plywood) that fits pretty close to the shape, with at most like a 1/2" gap with what you are trying to seal to. Install it as bare wood. Then in all the gaps, fill with some bondo or better yet, bondo glass. Take all the areas where the trim will meet the surface (usually a pillar, floor, or headliner) and cover it in painters tape so the bondo doesnt get all over it. Just smear the bondo in there, no points for neatness. Once it cures, take it out and sand the front and back flat, and wrap in whatever material you want to use for the trim panel. The bondo will have coutoured to whatever you smeared it up against creating a perfect fit, and once wrapped, it will look like you cut it out perfectly.
Its nice to use the hardboard because you can slightly bend it to get it to fit into tight spaces.
