I made a male foam mold for the seatstays, and vacuum bagged it with wetted-out carbon fiber cloth.
Here's the part during the curing process:
And here it is out of the bag, after one layer of carbon. Unfortunately, the bag did not conform to the shape fo the tubes as much as I had hoped, so there were a bunch of wrinkles in the part. Also, in the places where the two sides of the bag meet around the edge of the part, the vacuum pulled some fabric into the seam and away from the mold:
When I sanded down the seams, I found that there were gaps under them:
In places where the bag didn't wrinkle, the surface finish was really nice:
Despite the defects in the part, I decided to go ahead and add more layers of carbon fiber. There were fewer wrinkles the second time around, but there were still long voids along the seams. Also, I really overbuilt this part. The tubes are much wider diameter and much thicker than they need to be. I plan on remaking this part with a somewhat different technique, to make it lighter and less bulky.
When I made the chainstays, I did not leave enough clearance for a tire. I cut out a profile to make room for the tire, and then covered the gaps in the tubes with two layers or carbon fiber cloth:
Finally, I inserted a carbon fiber shim to bring the I.D of the seat tube down to the appropriate diameter for clamping a seatpost. The shim was yet another piece of the oar, that just happened to be the exact dimensions I needed: