Got part two of the saddle mounting brackets made yesterday.
This bracket has the single machine screw that holds the whole set up in place. Seems like a lot to ask of one 10-24 screw but I'm hopeful that my plan will work in the end.
The latest bracket on the underside of the carrier will fasten through the frame at the 'bridge' across the seat stays. That bridge would normally be used for mounting a fender. You can also see the notches in the fiberglass that hook over the bridge.
The two brackets (so far) bolt together with the carrier sandwiched between. The 8-32 screws will be upgraded to 1/4-20 bolts after everything is finished. I'm using the small screws at this point so that in the event that I have to make adjustments to the hole placements, there will be enough fiberglass material to move a hole without opening up a larger hole than needed. The extra holes in the bracket were from a miscalculation. There was a lot of eyeballing and guesswork involved in making everything fit the way it needed to.
This screw will hold everything in place but wont actually have a lot of stress directly applied to it. All of the rider's weight will instead be transfered through the steel brackets to the frame, not the bolts.
There are still more parts of this yet to be made.