John, I've just done some empirical testing on my Mk3.
M5x12 hex head screws drop in this way with ease. Allen heads would require a short reach key, but otherwise straight forward.
Jags has done it too, and Macspud's way would also work.
How does it help the OP to say a practical, low cost and neat suggestion is impossible?
As for design, Andy has given us double ended bosses that will work with the OP's caliper if the second most obvious assembly method is adopted. Hardly a fair criticism to call that a flaw, surely? Should I complain about rack fitment issues and lack of a cable stop if I chose to fit centre pulls?
Curious
Ian