The OLFA L-1 is my favourite utility knife. They're not expensive, but it's still the kind of tool I don't lend out. I have two of them, and one of their shuttles broke.
Once you've 3d printed them, jam a socket-cap M3 bolt into the knob, and a nut into the shuttle. If you heat them up a little, the plastic melts and gives way a bit, resulting in a nice friction fit. You could probably use a bit of epoxy or superglue for extra permanence too.
Mine lacks the satisfying dedent as you turn the knob, but otherwise works a treat. Feels solid and strong.