I managed to smash another miniblind cord cap and used white PETG this time around:
It’s the same solid model as before, sliced with whatever improvements have transpired during the last four years.
Made four of ’em, preemptively replaced the cap on the adjacent window, and tucked the last two away against future need.
The Smell of Molten Projects in the Morning 
These caps look like standard electrical wire nuts, with a hole in the top. {You can buy them with the hole already there.} Did you consider this low-cost alternative?
Of course not! That would require an in-person trip to a physical store! [shudder]
In truth, having already solved the problem (admittedly, the hard way), it’s now easy to Fire The M2, putter around the Basement Shop for a while, tie a pair of shiny-new caps on the cords, and move on.
Living in the future is great!