I was in a mood and drew this up in a hurry. Turned out alright I think.
There are 36 spaces, each one is 12.5mm x 12.5mm to hold a variety of things. The larger space on top will hold most pencils/pens/markers/tools about 165mm or less in length.
I've included an IPT of the model (Sorry folks, I work in Inventor) in case someone wants to make some changes for themselves.
In retrospect, I probably would have added more squares, and maybe shortened the height of the big top "cubby" space. I use the space, so I'm ok with it.
The screw holes are 4.6mm to accommodate a (what I believe is) #6 screw.
I also used it for a few days flat on a desk. Works well this way too. I'll try to upload a file with a back/bottom on the large space to it can be used that way as well.
**Note, I screwed up the orientation in the stl. Make sure when you put it in your slicer, that you place the largest flat surface (with the screw hole tabs) on the bed. Then you don't need any supports.
Make sure to use some setting along the lines of gap fill or similar. The walls are only 1.5mm thick, so if you don't do this, there will be a small gap between the walls.