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

This mount can be used on ToughBuilt Toolboxes. I designed it after buying myself a StackTech rolling toolbox to hold and move my tools when doing jobs for family and friends. It has two M3 holes that can be used to attach your own accessories. These holes are 41.5mm apart and have slots to hold M3 nuts at the back. 

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. 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.

I've designed accessories to attach to this mount that I will be publishing on Makerworld.