Buffer back plate

Resized the holes so the original screws can be reused just by threading them through, also reduced thickness to 2mm and removed the extraneous loops in the previous design.

There's no issue in just having the buffer be “floating” and it allows you to shorten the PTFE runs CONSIDERABLY, speeding up filament changes. Following DzzD's ideas: https://github.com/bambulab/BambuStudio/pull/1660

I didn't use top surfaces to save a bit of time and filament as this is not load bearing at all.