Oil filler door:
1. Carefully finished sanding the edge filler around the door. I am pretty pleased with how this came out!
2. To improve the fit even more, I ended up making two small improvements that sanding alone could not solve. I bonded a 0.018" thick steel shim to the face of the aft latch arm, and I fabricated and bonded a similar strip to the bottom lip of the door cutout in the upper cowl. These two projects really made the door fit as well as I can make it fit.
3. I test-fit the filler door spring, and the door works great! (Note to self: Be careful when removing the door hinge pin while the spring is installed. That little sucker can shoot at least 20 feet across the shop if you're not careful.)

Lower and upper cowl improvements:
3. To get the lower cowl to look good, I continued filing the outside of the lower-aft corners, where they curl underneath the fuselage. Then, to beef up this area once again, I added an epoxy-flox mixture on the inside of the cowl.
4. Sanded the previously-added built-up flox areas along the inside of the top cowl, where it meets the firewall.
5. Block-sanded the rivet lines on the lower cowl so they are super-smooth.
6. After looking at the lower cowl with a critical eye, I came to the conclusion that the cowl exit lip seemed somewhat flexible and floppy, especially for such a high-stress area. It looked like the relatively thin fiberglass there might buffet a lot from the exhaust pulses, the exiting cooling air, and even just the ambient airflow under the airplane. I decided to strengthen it a bit. I laid up a two-ply of BID cloth and high-temperature Aeropoxy, and laid it along the inside of the cowl exit ramp. We'll see if this makes it significantly stiffer. If not, no harm / no foul.
7. Removed both cowls. Beveled the edges of both the upper and lower cowl. Trimmed some previously-marked area on the lower cowl.

Other than installing a few nutplates and filling all the pinholes, I think the cowl is nearly done.