Update: all the XY ends done + small improvement

All the XY ends are printed, ready for installing.
After some thought, I reversed the direction of M8 bolts. Now the bolts sticks from the inside out. This has several advantages:
1. Nothing inside the plastic part has to rotate when I'm tightening  the nuts. So it's much easier to assemble and disassemble.
2. The structure is less sensitive to the bolt size.
3. I think the nylock nuts look way nicer than bolt heads.