A Gridfinity base plate that I made to fit an awkward tray in my desk.

The larger grids are the standard 42mm x 42mm, and the smaller grids are 21mm x 21mm

The model measures 168mm x 63mm x 4.65mm and should fit on almost any common printer

This base plate is designed to that it can be placed on top of a thin steel sheet, to allow unorthodox placement of magnets, which is particularly useful for the 21mm grids, as modules are not large enough to fit four 6x2 magnets well. 

21mm grids work well for holding single items upright, such as an exacto/hobby knife

Any standard Gridfinity module can be scale down to 50% to fit 21mm grids, however modules specifically designed with this scale in mind are recommended. I'll do my best to upload any 21mm specific grid modules I create.

Note:  While the corners for the standard 42mm grids are filleted, the  corners of the 21mm grids do not have filleted corners due to how thin the walls already are. This doesn't harm functionality in any way, but is worth mentioning as some may find it annoying