what is the aluminium grade and thickness of the adapter in the last photo? (the thickness of the single stepped part, not overall thckness)
without going back, I think I saw mention of 3/16 " = 5 mm or so - is that going to be strong enough?
I have made rear brake adapter from 6 mm aluminium, not sure of grade but judging by colour is plain raw aluminium .... but looking at disc brake adapter ansd drop out, these tend to be 8 mm to 10 mm thick so my one may not be strng enough (I am in process of making another out of 8 mm).