A trick I tried with the latest plane for mating the cowl to plane worked wonderfully. I got the basic fit done and set up the hold downs and such. The I finish sanded the cowl and go it pretty much done. this a glassed the cowl with half ounce fiberglass and epoxy. Sanded it and polished up up to a mirror finish (well, at least a very smooth finish) The surface that faced the plane was also finished very flat and was glassed. Once that was done, I waxed the cowl and shot it with PVA release. then I put Superfil on the mating surface on the plane and bolted the cowl on. I made sure that any gaps along the mating surface were filled and smoothed. Once dry, the cowl popped right off due to the wax and PVA and I ended up with a perfectly mated cowl. Virtually no cowl line.
You do have to insure that you get all the PVA and wax off the cowl, cleaning it well and re-sanding it after your done, but it sure worked well. It's a trick I'll do again.