I think I found the solution.
I bought a couple of adapters that came up with a sort of clamp. The adapter itself measure 4mm and it is made from stainless steel.
I was thinking to remove the clamp and gave to a professional who can solder directly into the frame and fork.
At least I cut the cost and also the time to find a piece of steel and making the piece itself.

​​​​​​Probably I'm going to reinforce them a bit too....