Decided to make a holder for the most common sizes of post-it blocks, i printed with the "left" side down. 0.2mm layerheight, 0.3 nozzle and 10% infill.

Comes in three different lenghts (All dimensions is in millimeters):

Small:
38 x 51 x 10
Normal:
76 x 76 x 10
Large:
127 x 76 x 10