My initial goal was to simply remove the attachment tabs of the base, but Tinkercad had problems with the model, so in the process of cleaning it up, I made some other adjustments

• Remove tabs—this requires that the inserts be glued to the base.
• Resized from ~84mm to 83.5mm in both X & Y, to better coincide with Gridfinity specs (note that I did not adjust the top magnet holes, so they might be off by 0.25mm)
• Redefined the base

FYI: I sliced with Cura 5… none are likely required:

• Layer height: 0.28mm  (for speed)
• Infill: 10% cubic infill, cubic subdivision for strength and speed. I do not recommend Lighting… it'd work, but it doesn't feel as solid
• Horizontal Expansion: -0.2 to make sure the opening is printed no less than the model definition (since its somewhat precise)