This part is made in two pieces that nest. Print each piece in vase mode (0.4mm nozzle, is what I used), then slide them easily together. And you can store up to 15 endmills, 1/8in. shank, neatly in one #gridfinity square.

The trick is printing the base/outside piece right side up, and printing the insert/endmill holder upside-down. Set your slicer for vase mode, and print them one at a time with a 0.4mm nozzle, and they should slide easily together.

The trick to designing such a part is making sure that there is only one path around the whole thing, that's why the side lines are cut in. Also, at the bottom of each hole is a slant which allows for minimal stringing as the hole tapers to a single straight edge.