I needed a box to hold the screw set I bought for use with T-Nuts - and of course this box had to use T-Nuts themselves.

This one fits a box with 100mm x 24mm and uses M2.5 screws with 16mm length for the T-Nuts. If you don't know much about T-Nuts yet, check out this link: https://www.printables.com/model/228663-skadis-t-nuts-mounting-system-for-ikea-skadis-peg
According to the calculations in that link, any screw length between 12.4mm and 26mm should be fine.

I designed this with FreeCAD (file attached) and used a VarSet to modify the most important variables - you can change the screw diameter, screw size, and of course the size of the box it should fit. After printing my box I noticed that front holes for easier access to the screws would be helpful, so I added them now. You should be able to remove them using FreeCAD easily.

Update: I added a smaller one to hold my mints tinbox (inner size 98mm x 22mm), and it was super easy to attach to the Skadis wall now, thanks to the front holes. Same screws as above, I added the files as the -smaller variant.

If you remix this using my FreeCAD file, please upload it as a remix and tag me. If you need a box in other dimensions but don't know how to use FreeCAD, ping me and I can see if I can quickly generate a new size for you.