I wanted a custom size tray for bolts that would fit on the arms of our 2 post lift, I made this model where the bottom footprint is 10cm x 15 cm with 2 heights, approx 15mm and 30mm.

Inside is 12 x 6mm x 3mm magnets which one can buy cheaply on aliexpress or similar.

I grips very well to metal surfaces.

To print add a filament change at the correct layer height, place the magnets inside the model and continue printing. This ensures the magnets stay in place and keeps any metal shavings on the outside.