Single Edge Blade Holders

UPDATED 2018-08-01: I closed off the "back" (opposite the finger rest) so the blade would not slip out the back if you are pushing forward when cutting. Insert the blade in the slot under the finger rest and push it to the back where it will stop. (Idea is thanks to a Thingiverser who noted the blade can slip out the back when pushing forward.)

MAIN NARRATIVE:
These are two single edge razor blade holders that are identical except one has ribs for gripping and the other has an indentation f