I cut down the foam into a block for the spot where the chainstays connect to the bottom bracket, and two tapering rectangular rods for the chainstays themselves, and glued the three pieces together:
Lots of sanding ensued, and the end result was this:
I didn't think that the foam itself was sturdy enough to not get warped when vacuum bagging, so I laminated over it with a layer of fiberglass cloth using the electrical tape method. The fiberglass also insulates the carbon fiber from the aluminum.
To do the vacuum bagging, I made a nozzle that connects the vacuum pump to the bag. The barbed brass fitting threads into the circular aluminum plate through a small hole in the bag, and squeezes the bag between the washer and the aluminum plate to seal it.
The fitting worked reasonably well, but I was never able to get the bag sealed enough to retain a vacuum. This would not be a problem if I could leave the vacuum pump running for ~10 hours at a time, but apparently the MITERS vacuum pump isn't designed for continuous use.
To laminate over the chainstays in carbon fiber, I chose to make my own unidirectional carbon fiber cloth out of the giant ebay roll of 12K tow I bought for my bamboo bike, rather than begging the FSAE/Solar EV teams to give me some of their cloth.
These sheets were folded over into four layer thick bundles, which cover the chainstays something like this:
After soaking everything through with epoxy, they looked like this. One leg has already been sanded:
The dropout area will be further reinforced:
As will this bit. I may neet to carve away some material and re-laminate it, as I'm not sure I left enough room for tire clearance.
Over all, it worked pretty well, although using tape to compress the joints made the surface wavy. Lots more cleanup work will be required to get them as smooth as I want. I have also found a few spots that are too thin, and will need some more reinforcement. Once I finish cleaning up the surface and reinforcing everything, I will probably do a thin cosmetic layer of carbon to make it nice looking.