I mucked around a bit and worked out how to seat the front of the door properly, and also seat the bottom sides of the windscreen.
For the door, a Z-profile bit of 0.025" does the trick, providing a narrow sill for the front to go into. I've put a small self-adhering rubber along the inside of the door, with holes cut where it goes over the rivets, and the door is pretty tight so I'm hoping it's fume-proof to some extent.
For the lower sides of the windscreen, a flat piece of 0.016" cut to shape is riveted to the fuse below the edge of the windscreen. This has turned out quite neat so I don't know that I'll even need rubber.
I've also test fitted some black felt above the dash, still need to trim then stick it on with spray adhesive before riveting & screwing the windscreen into place. Ripped off the protective plastic too - for the first time I can sit inside the plane and look out through the screen, I really need the internal windscreen wipers for the "brrrrrr" noises now...