Allowing for frame modifications..

Run a piece straight through the frame, for the bosses you attach the shift levers onto,
they typically as you see, on the steel braze ons, have 2 flat spots , those flats for the outer friction plates,

those flats could be also keeping the pieces that you bond onto the Frame tube with the lever stops ,
from rotating relative to the outer friction disk

typically fit over a Square base of the Braze on , to
stop the lever from Moving past the forward slack cable .. stop..

The difference between the flat stop which the lever uses for part of its friction
And the round of the Tube can be done in something like Epoxy Putty.

the pins as suggested above would just penetrate enough to keep the plate from rotating.

perhaps just the bonding by the Epoxy would handle the lever and friction shear forces , IDK ,

I'm just working with mechanical practices and thinking of practical Materials, to use..




in addition.. if you can Make it in Wax , you can get it cast in Metal, even Aluminum.