Hi DIYTinkerer, I've printed both the 1x1x1 hex bit holder, and this 3x1x2 hex bit holder. I found the fit of the hexes on the 1x1x1 to be absolutely perfect, positive grip without any effort. I can tip the 1x1x1 upside down and no bits will fall out. However, on this 3x1x2, they are a bit looser, and bits will fall out. Is the fit of the 3x1x2 more relaxed, to incorporate the comments that LengAwaits made? I had also slightly changed the printer setting between the two prints, so wondering if that may be the different instead? Regardless, appreciate both models!!

I made changes as suggested by LengAwaits and also added holes for magnets and screws

Forewarning to anyone looking to print this file, and a constructive criticism to the designer:

The tolerances ended up being too tight for my machine. While my machine isn't tuned to absolute perfection, I am able to print most print-in-place and mechanical parts, requiring fairly tight tolerances, without issue. My hex screwdriver bits measure 6.31mm from flat side to (parallel) flat side, and measure 7.17mm from vertex (point) to vertex (point). Based on the measurements I've taken in 3D modeling software, they should fit perfectly in this model, and with a little wiggle room, too! Unfortunately, they don't. The vertex-to-vertex measurement is the culprit, for me, as the side-to-side measurement is wide enough in my case.

Of course, this could very well be an issue with the machine I used for the print. It's the newest Ender 3 Pro (bowden) in my ever-growing collection of machines. Upgraded with a cr-touch and an SKR V2 Mini mobo. I wouldn't be surprised if it's overextruding a bit more than I'd like when starting the outer (hex bit facing) wall, or while cornering on each point of the hex cutout. Rather than the 7.51mm they measure (from vertex-to-vertex) in software, my print generally saw them at less than 7mm on average. Side-to-side measurement, in software, is 6.5mm, and as mentioned my print's side-to-side hex holes are certainly wide enough, measuring around ~6.51mm on average.

Long story short, make sure you're well dialed in, otherwise find another Gridfinity hex holder. I've had good success with one that measures, in software, 6.93mm from "flat to flat", and 8mm from "point to point." It's made by beakerz. Unfortunately he doesn't have a version without a tray, and the bits do wiggle a little bit. I'll take that over wasting plastic any day, though.

I didn't want to be arsed with making one myself, but I might take a stab at it eventually. These tolerances seem to need to be sloppier for your average 3D printer owner. Myself included.