A customizable parametric oil filter wrench. Create 3/8" drive or 1/2" drive filter wrenches of any size you need. It's not pretty, but it's designed to be easy to print and strong. (Print it with high infill percent for strength.)
2017 03 29: Added 1/4" drive option as requested, but did not build or test it. Not recommended.
If you make a custom one and publish it, be sure to include info about what it fits. That way, we'll end up with a library of known good sizes.
Oh, and if you build one and use it, please let me know how it goes for you!
HOW TO MEASURE:
Count the number of faces on your filter. If odd, measure the distance from the center of one face to the vertex opposite the face. If even, measure from a face to the face opposite it.
(Do not measure vertex to vertex. Note this instruction has changed.)
Note that the code will add a small amount of slop to your diameter (0.66mm).