Inspired by this model, I designed this from scratch to print easily w/ no supports and with easy assembly. 

Print both sides for a +X/+Y counter marker, print just the hole side for a +X marker. 

You can scale these as needed in your slicer, 100% is nice and compact, 125% is medium, 150% is a nice chonky one that will be easier to read from afar. 

As pictured in the cover, printed at: 
150% scale
.2mm layer height
15% grid infill
3 wall layers
No supports
No Brim