CR-10 Stronger Squash Foot

I liked the idea of Dr. Boo's squash foot, but I did not like the overhang between the sphere and the flat plate. I threw together a modified version that adds a fillet and some gussets in that location. This version is probably a bit stronger.

I also moved the mounting holes out to 50mm apart. This should make it a bit easier to install the foot, as the ball no longer shadows the machine screws.

I used [double yellow dot squash balls](https://smile.amazon.com/Tecnifibre-Double-Yellow-Squ