The squares hold the plastic cases that ¼" endmills commonly come in.  There are holes for “naked” ¼" and 1/8" endmills.  The slot on the right is for a collet wrench (fits the OpenBuilds one for sure).  The larger holes loosely hold empty ER11 collets.

I cut this out of MDF on my CNC, but 3D printing is an option as well.  If your printer prints small, not-so-round holes, make sure you scale the entire thing up in X and Y by a few percent to allow for clearance.