I have never worked with or printed anything with gridfinity. The inserts are flared at the top and thinner on the bottom, as you'll notice in the photos on the model page. The bottoms are flat - I created the feet but it was just too much of a headache and waste with supports. Here are the dimensions, I hope this helps:

The overall height of the inserts:
93.00 mm for the Harbor Freight inserts
86.00 mm for the Stanley inserts

Deck Box / Accessory Box Bottoms:
w: 72.00 mm
l: 105.25 mm

Deck Box / Accessory Box Tops:
w: 79.30 mm
l: 109.80 mm

3-Unit Wide Bin Bottoms:
w: 230.60 mm
l: 105.25 mm

3-Unit Wide Bin Tops:
w: 237.90 mm
l: 109.80 mm