I have a bicycle rail for the roof of my car, made by thule. It has a tool-free assembly system with nuts necased in plastic handles for fixing on the traverse bars. After some years the original plastic holders started cracking. I strenghtened one of them by wrapping it with some wire, but still made me feel a bit insecure. This spring another one cracked wide open. Since I recently aquired a 3D printer, why not try and make a holder by 3D printing?

My printed handle has already taken over the job on my bike rail for a month and a half now, including some very hot summer days. Tight, firm, with no sign of having been affected by excessive heat :-).