Locking blade utility knife with clip. Uses standard utility blades. You will need a little screw to retain the blade.
SolidWorks files attached.
Print handle with the cavity pointing up. Print blade holder with blade slot vertical. Print pin standing on end.
Use a utility blade to trim off the "locking tab supports" and shave down the pin to fit (it may be a 1/4 mm thick depending on your plastic) Assemble the knife, and fix the pin in place with a little "ABS juice" or squirt a little Goof Off on it. Insert the blade, and attach it with an M3 x 8 screw (or similar, it's very forgiving)
Enjoy your one-handed-opening, pocket clipped, ultralight sharpness!
Possible re-designs: find a way to affix the blade without a screw. The thumb-stud could probably be reworked to gain this feature.