5x2 Gridfinity tray that'll sit nicely in the left recess of the CORE One. For stashing tools, nozzles, sheets, Haribo bears or anything else you want.

Either print as one piece, or use the separate base and tray to reduce the need for support and allow for two different colours. 

It's a tight fit to get this printed in one piece. You can just about fit this on the CORE One if you print it front side on the print bed and turn 40 degrees around the z axis (it should import to your slicer already in that orientation). Enable support (on build plate only, if you're printing the two parts separately), but you'll need to change you Print Settings > Support material > First Layer Expansion to be ≤ 1.5mm

For the magnets, i sized this to use 20 x 6 x 2mm, which are the same as Prusa supply. (But I find it quite difficult to source magnets that size, so I'm using 20 x 6 x 1.5 mm myself held in with a drop of glue). There are spaces for 14 magnets, but that might be overkill, I seem to get away with 8.

If you print it in two parts, the tray just drops into the base. 

(If you're interested in the sheet sheet holder pictured, the model is here)