Here I thought I was being clever! I should have figured someone else figured it out before me. In my own version of this mod, I drilled through the horizontal ribs with a 12"-long 1/4" drill bit and passed the allthread through all the holes. Next, I filled the first and last segments of the channel with AcraGlas and fiberglass floc. Finally, I filled the middle two segments with an AcraGlas/microballoon mix.

I also laid up a few layers of fiberglass fabric using epoxy on the outside of the forearm, but I'm dubious of how much that could help. I may try it again with slightly different approach.