In my garden I have some long rebar that are I attach trellis to. Now that my kids are walking I worry about them falling into them. I wanted the standard orange caps you see at construction sites but all I could find at the big box store were 4" square overkill things that still had sharp corners and cost $4 each. So I printed these.

My rebar is #4 (1/2" diameter) but in reality they vary quite a bit and some have elongated seams along one direction. So I made the opening a 9/16" x 3/4" oval. 3 out of the 4 ended up being a nice snug tight fit. The 4th one was loose so I heated up some InstaMorph and put it in the hole before shoving it on the rebar. It seems to hold nicely.

The standard ones you see at construction sites have fins that bend to grasp the rebar. I didn't feel that my ABS was ductile/elastic enough for that.

I modeled it in AutoCAD 2012. I tried sketchup but wasn't happy with the fillet options.