Ultimaker Original mount for low friction spool holder

I had some issues with the orginal design, but it was a great starting point. Thanks, fns720!

1. Modified the position and thickness of the legs that log in to the spool mount, hopefully accommodating more variance in the wild.

2. Added a groove down the middle to accommodate the nuts/bolts that hold the wooden mount in place allowing this spool mount to sit flush.

3. Enlarged the hole that connects through the mount to the m6 nut. Reducing the chance of delamination when threading the