After assembling the metric Hadley and trying to collimate, I noticed the collimation knobs hit the tubes:

Since I really want to have the tubes extend a bit further so the OTA will never rest on the collimation knobs, I made a slightly smaller version. I also made the whole knob a few mm shorter, giving me a lot more room for collimating using 40mm screws in the primary mirror cell. Now, the knobs turn fine while still having a satisfying feel to them: