Don't you hate it when you can't find a capo, and you need one?! Fortunately, this is a quick print, so you can get it going fast! This is a clamp-style capo with a screw tensioner at the back, so it's pretty foolproof, and can be printed in just about anything (probably even some flexibles! give it a try!)
Size-wise, this is geared to fit the banjos, mandolin and ukuleles I have lying around, both in terms of the neck width and the depth. It won't fit a guitar, but there are plenty of existing capo designs for those!
You'll probably want to put something between the rear tensioning bolt and the neck of the instrument, since grinding hard plastic against delicate wood is not going to go well in the longer term.