When I said the button shafts were smaller, what I really meant was larger...

So the RS buttons are larger around than the holes in a Happ thumbswitch plate. Make it a little bigger and they should work, or there are always the blanks. Good thing I just bought a 1/2" bit.
In other news, I just received an old Williams 49-way from Lilwolf, will do some playing around with it. One thing that seems promising: The spacing of the mounting holes on the spider plate look to be the exact spacing of the screws holding the bottom of the Happ 49-way together. A longer set of screws and a repro of the old plate would make mounting a stock spider to the Happ stick quite easy. The only problem is the small size of the hole in the center of the spider.
I also received today a repro spider (home molding job) that has no metal inserts as the NOS ones do. It is very stretchy (easily stretches to 200% of its normal length without stressing) but seems stiffer than the NOS spiders, which might make them well suited for the extra leverage exerted by the Tron shafts. The center hole is easily stretched over the shaft of a Happ 49-way without any major distortion, so I will have to do some durability testing to see how it holds up.
One drawback to the repros is that they have a LOT of bubbles in them, which could turn into tears pretty easily. I think the best approach will be to have new ones cut from solid rubber with a CNC water jet as I had first planned. I will also look at prices for injection molding, but the setup tooling on molds tends to be high.