I do believe that the F106 and F106/1 can be very highly stressed in service. Generally, they are not put to hard work except in racers or sidecar machines. If the plates on either side of the F106 are not well-designed or improved, the item will crack in several places. This is unlikely to occur on a street bike, but if you are planning to use the bike in competition, it is a concern.
The Stilltime photos of the Grey Flash prototype show steel tubes used instead of the F016.
This set-up would not be very rigid, but considering the tire technology and the track conditions at the time, it appeared to be a suitable choice.
I think steel could be a good choice if fabricating an F106 is necessary and sidecar or competition work is contemplated. Using a light steel box construction would appear to be a good choice if something special is desired. Grade 8 bolts seem to be mandatory. The 6061 plates seem to work fine, but I use 3/8" width, not the original 5/16". I use 7075 on the primary plate.
Weakness in this area can cause chassis wind-up, rear damper breakage, F106 breakage, RFM twisting, and gearbox shell damage. All of this is easy to prevent through a more robust design, but it does add some weight. I have always believed that this is the best area on a Comet to be overweight. The twin does not have this problem as it is unit construction.
David