I was in need of cutting strings to length,  and found the String Cutting Jig by OurAngryBadger:  https://www.printables.com/model/58013-ribbon-string-cutting-jig

I was going to try to tape it somewhere and realized that I could make something to hold it.

First I created a version in Prusaslicer that works well, but then decided to make a customizable one in OpenSCAD.   The code isn't great but it works.   The string distance is the distance from center of hole to center of hole,  so your string will end up slightly longer than the distance specified in the file.  For my purposes, that's perfectly acceptable.

Pretty much everything is customizable if you want to use this for some other purpose.  And I'm sure there's much better ways to do this.