Bear 2.1 Alpha T-slot

Modified key parts from the latest alpha of mk3 bear upgrade for misumi t-slots.

I left spacing for hammerheads in most but so far its working fine w/ the flat misumi nuts.

Original and additional files for 2.1 alpha are over on pekcitron's github. https://github.com/gregsaun/prusa_i3_bear_upgrade/tree/dev

Additional T-slot modded parts for 2.0 some of which can be used for 2.1 and inspiration for remix from Jason's T-slot remix. https://www.thingiverse.com/thing:2872569

Also modified the holders for the lrs-350 hinged psu holder by fasteddy516. https://www.thingiverse.com/thing:3450424

And the tensioner by RH Dreambox, though i ended up preferring pekcitrons version. https://www.thingiverse.com/thing:3502543