Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:3853087.
If you use a glass plate to help you tram your CNC per the various Youtube videos out there, you may currently use playing cards or such stacked at the corners to build up a level plate from which to tram your CNC cutting head. I've designed this to be placed at each of the four corners of the glass plate to help level the glass plate prior to tramming.
The adjuster is designed to fit a 3mm glass plate and house 3mm nuts within the cavity at the outside edge with a 3mm screw threaded down through it. The premise is that you would install one of these at each corner with each screw threaded about 1 mm out through the bottom and then set the plate on the CNC bed. You would then run an indicator inserted into your spindle's collet to one corner of the plate where you would set the indicators zero. Then run to each of the other three corners and adjust the screw up or down until each is at zero. When all four corners are zero on the indicator, the plate is level and you may begin tramming your spindle in relation to the plate. The bed is then surfaced.
I hope this helps some of you. It's saved me a lot of time such that I don't hesitate to tram my spindle every so often anymore and no more messing around with shims or playing cards.
I've oriented the model the way I find is easiest to clean up (the little square hole for the nut is a real pain if it's printed horizontally) but you can orient it for how it's best for you.