Edit 1: FWIW I originally thought these were 20A cords, but they are actually heavy duty 15A. The files still say 20A and I'm not changing them :-)

Edit 2: I made several of these spools into winders no problem, then ran into a couple that were slightly smaller (~196mm vs. ~200mm). If you run into a smaller spool you'll have to scale everything down slightly. I suggest printing one of the side hole inserts before printing the whole project to make sure it fits or to find the right scale.

I sell a few 3D printed winders for antenna wire, coax, cordage etc. like this one https://dreamlabnc.etsy.com/listing/1684675942/winder-reel-475-120mm-diameter-reel-for and set out to design a bigger one for some 15A heavy duty extension cords I have, when I looked up and saw all these old Anycubic plastic spools I've been saving for some unknown reason.

They were the right size for 30 feet of cord but I needed to turn them into a proper winder like my other ones, so I designed some inserts for the holes and some clips for the cables and BOOM!

The center insert has just enough room to get your hand in there and be the axle for easy winding, the finger hole let's you wind easily and the clip inserts lock down both sides of the cord.

Included are insert clips sized for a 15 amp heavy duty power cord and one for RG-8X coax, both with single and dual clips. I also separated the inserts and clips so you can easily scale the clip to whatever size you need and mash onto a blank insert clip in something like Tinkercad.

Print in PETG, will make it easier to flex and squeeze the inserts into place. Put one side in and angle the shorter side a little bit and they should snap in fine. For the center hole insert, insert the cut end from the back side (opposite side of the finger hole and clips) by gently squeezing one of the cut sides inside the other side and it should go through and snap into place.