For our Christmas lights, we use gutter clips, but they get brittle in the sun and break. They had very small prongs and could not be used more than five years (picture is included). Mine have been there for half a year and are still very pliable(I will try to get back to this model sometime to see if any of them are breaking).

edit (10/19/23): mixed results summer of 2023 most were still good, but others had deformed into the state they were stretched into. 

Petg might do a lot better in the outdoors. Definitely worth a try.

advantages:

• they can be printed in any material (I think the commercial ones were ABS)
• they are easy to print with flat faces
• they can be any color (not just the white they are sold in) meaning they can blend in better
• works with most, if not all, light strands

printed on my ender 3 and qidi x pro with black ABS(TPU definitely does not work)

I recommend using 2 walls and 100% infill, but you could use less infill.

If you sell them, just mention me.