These are replacement HBP corner pads for the Makergeare M2 that secure the build plate with spring force, eliminating the binder clips. Use with either the stock glass build plate or a Zebra Plate. Update: I'm now using this also with a beadblasted MIC6 1/4" aluminum plate (from jimc of the Makergear forum), with chamfers filed into the plate. This works well.
The two needed springs are McMaster-Carr part number 1986K64. These are optional if the parts are printed in polycarbonate (esun ePC or Polymaker Polycarbonate-Plus).
A complete parts list and assembly instructions are here: http://lightsmithscientific.com/justine-haupt/M2/DropInBuildPlate/MakergearM2DropInBuildPlate_AssemblyInstructions.pdf
Material note: I originally made these in PETG but have since discovered Polycarbonate (esun ePC) and have reprinted them in that material. The photo of the parts sitting on the bed are the new polycarbonate ones. The black ones shown in all the other photos were PETG. Also, it was observed on the Makergear forum that printing these in polycarbonate may be necessary for long term resistance to the 100C bed temperature used when running with polycarbonate anyway.
See assembly instructions for more detail. These were originally printed in PETG, which worked well, and reprinted in ePC (polycarbonate) to run the bed at very high temperatures.
It appears that the McMaster springs are quite unnecessary with the ePC versions. I'll wait a few months to be sure they're not loosing their springiness before being sure the metal springs aren't needed.