This is push pin plastic clip or rivet if you want to call it that way. It is for 8mm holes, but you can scale it to any size you need. It took me few hours to make it precise. To make different diameter use this: YOUR HOLE DIAMETER ÷ 8 × 100 = YOUR SCALE IN PERCENTS.
Original SketchUp file included (.skp).
Added 25% longer versions - longbody.stl and longpin.stl.
If you need to design part in stl, send me email to firstname.lastname@example.org. I am not making organic shapes like statues, animals etc. I can make various holders, brackets, boxes and other technical stuff.
Made in SketchUp from scratch, took me few hours. To use, put body in hole, than put pin in to lock it.