Anycubic I3 Mega Spool Holder Remix with filament guide

This is a remix of "Spool holder for Anycubic I3 Mega" by kurioso.
I really like the original, but the parts were a little loose and I had problems with the spool not staying on and filament getting tangled.

I've made the following changes to the arm
- moved the spool closer to the Z-axis mount
- tightened the z-axis post clip so there is less play
- thickened the spool holder arm slightly so the cylinder fits tighter
- modified the brace to be at w 45 degree angle rather than horizo