This is for the Biltema U-lock, though I assume many U-locks/ wire lock use a same holder. The original holder didn’t fit on the 40mm pipe, so I made one myself.

I used the original bolts (2x6mm m6) and a spring from an old pen (maximum 4mm in diameter). The spring length can be trimmed to fit, and I think 15-20mm should work fine. 

For the locking mechanism to hold the button in place I used a piece of filament, 33mm long, which I pushed into the small hole then heated the end and pressed to secure it. Alternatively you could widen the filaments end with a plier before pushing it into the hole to act as a locking pin. The hole depth is 30mm.