Prusa Filament Buffer Plate - Enhanced and Printable

# Statement
Prusa released their "Filament Buffer" with the MMU2S update but there was no STL for the plate on at [MMU2 GitHub](https://github.com/prusa3d/Original-Prusa-i3/tree/MMU2). There is a file named s-buffer-plate.dxf in the SCAD directory but the plate is too large to print with the MK3 and they had it manufactured, thus no STL.

The buffer is a necessary update but I found it quite difficult to push filament from the buffer to the MMU. I have several brittle filaments that were almost impossible to load without breaking. The only technique that worked was fingering them all the way between the buffer and MMU. It was clear to me the shape should be rounded and provide just a little more room.

# Solution
After seeing the buffer in action I could see no reason why a slightly smaller version wouldn't work just as well.
I was desperate for more fingering room