I use a lot of 1515 (that is, 1.5" by 1.5", not 15cm by 15cm) extrusion.  Sometimes I need to mount things to it like lights or low-voltage stuff.  And sometimes sometimes there are reasons why I need to include extra wire as part of that.  Sometimes sometimes sometimes I need to keep things very neat and clean.  On those occasions, I use this: a covered wire spool which attaches to 1515 extrusion.

These are light-duty pieces, intentionally, so I decided to print the mounting hardware instead of using metal.  That part of things is a little messy, because you'll need to clean up the threads on all of that stuff post-printing.  That's easy enough to do using a properly-sized (¼"-20, in all cases) nut or bolt or tap and die set.  You can also skip printing those parts and find some metal hardware that works.

The attachment holes in both the base spool and the top have a bit more slop in them than I usually include to allow some flex as this stuff comes together.  I find that to be useful in this case.  The cover is sized to be slightly narrower than the 1-½" extrusion width in case you've got bypassing parts.

I included both a 3mf file and some bgcode (for a Prusa Mk4) because there are some printing considerations here.  To wit:

• Infill should be in the 40-50% range for the attachment hardware, with four perimeters
• Infill otherwise can be 15-20% with two perimeters, but more won't hurt
• The hammer nuts need to be supported from the platen
• The spool needs internal supports; I've painted those on to avoid unnecessary cleanup later
• I used PETG.  Depending on your use case, other materials will work fine