AgX, I am an avid practitioner of DYI (do it yourself) and I have a pretty good collection of tools for working with wood and metals - but I do not own a metal lathe.
I assume José Meneses (Custom Photo Tools) is able to sell the adapter for 15 Euro only because his specialty experience allows him to make the part with great efficiency. I am guessing that a typical machinist would require maybe an hour to set up the lathe and machine this part from an aluminum billet if it was for the first time - yes? no?
I don't know for sure what machinist labor is worth, but most shops in "the trades" - electrical, plumbing, auto repair - consider an hour to be worth AT LEAST $60 (US), and $100-120 is more typical. And most charge for one hour minimum. So my assumption was that the part would be well over $50 from a local machine shop??
But thanks for the recommendation. I will call around to some machine shops on Monday in case my assumptions are wrong. I woud be delighted if I could buy the part locally for $20-25. Then all I would have to worry about is getting my money back from José.