UPDATE 31/3/26 - I can confirm that commercial M5 x 12 T bolts do work very well. I purchased mine as a 50-pack from Amazon for about $18 AU.

After buying a Toughbuilt rolling toolbox, I decided to design my own accessories. The hex bit holder can be mounted on the corner holes on Toughbuilt toolboxes, inside the lid of the top-opening toolboxes, or onto the mounting plate that can be screwed to almost any surface. The hex bit holder uses a simple clip-on spring-loaded adapter that I designed. The adapter can use springs from old pens, or spring/s with a maximum diameter of 5mm (you might need to drill out the hole if 5mm diameter springs are used). I've added a photo of a packet of springs I purchased from Bunnings in Australia that are perfect, but a little expensive. You can also buy springs from Bambu Labs, so I've selected a couple that should do the job too. The number of springs required will depend on the strength of the springs. The adapter uses M5 x 12 pan head bolts that have been cut or filed down, rather than 3d printed tabs that I found could break off - I'm waiting for M5 x 12 T-bolts to be delivered to confirm that they can replace the pan head bolts. The t-bolts just screw into holes.