As the "nut" which is a screw was severely screwed up at the tip in the original design (just look at the pictures or STL files) I fixed that in tinkercad. It is no surprise that the original didn't fit into the handle and that some people had to shrink it to 90% in X and Y. I also made the screw with 2 different inner diameters to better suit my set of hex-keys: 4 and 6.3mm. The STL files for these revised screws are attached and still named nut. I also included the original handle for convenience but it is only flipped to be directly printable without support and the slot widened to accommodate 5mm hex keys. My conclusion: very good idea but a bit sloppy design.

Side note: the screw with 4mm hole I shortened by moving it 4mm below the surface before slicing.

2022-04-14: on request I added handle and screw for up to 8mm keys (-9-5mm) and up to 10mm keys (-11-5mm) to the STL files. But caution! I didn't print them as I don't have a need for it.

2022-04-17 As soon as one believes something isn't needed a new toy shows up and it gets urgent. Well, I had a use for the 9.2mm handle and printed it. Works as it should which means this design modification is proven too.

2022-04-18 Found out why some hex keys tend to be not perpendicular held in the handle. It's the bend in the keys which interferes with the screw. That's why I chamfered the hole as much as possible. Files end now in -V2.