Flash gel holder for the rectangular gels from amazon (neewer 30 pieces kit or 35 piece kit - probably sold under many different names).
It comes with single holder - this is to make more holders if you have more flashes.
The gels are about 104mm long and 58mm wide.
It is made using two parts in order to make the necessary fraction of mm clearance (it would not be possible to print that obviously). You simply put the inner frame inside the outer frame making sure there is clearance between the tabs and the inner frame - just look at the cross-section pic. The frames should be tight fit - but if your printer is different than mine use glue or sandpaper - whatever is appropriate.
The B part needs to be printed with support.
You can then use velcro, rubber band or even tape to mount it on the flash.
100 % infill. (those are thin pieces)