I have fished furled leaders for several years now. At first I made them by twisting with an electric drill:
https://northbranchreels.com/2010/12/04/furled-leader/
Later, I made a hand cranked furling fixture: https://northbranchreels.com/2014/07/31/furling-machine/

A week or so ago I was contacted by Ben, a resident of my adopted town of Rio Rancho. He wanted to make leaders but was not prepared to make a machine like mine. So he improvised with the kid’s building set K’Nex. Here are 3 pictures of his furling head.

I am not familiar with K’Nex, but it looks as if the input gear here may be several inches diameter. Three high speed furling shafts run 14.2 times as fast as the input crank. (My speed-up ratio is 84/17 = 4.9 .) He also has a fourth spindle with ratio 5.9 .

His setup frame has adjustable post locations, so he is making not only leaders but Tenkara lines of up to 15 foot length.

As on my machine, he has a dead weight to maintain the line tension while furling. I think that this is an important feature.

How could you make a furling machine if you had no K’Nex nor any capability to make gears? I just did a search for “plastic gear” and came up with a promising supplier, http://www.gizmoszone.com/shopping/agora.cgi?page=plastic_gear.htm . The category “spur gear” seems to be all 0.3 module, which is a tiny tooth. But from “compound gear” you can find 0.5 module with up to 60 teeth. The high speed gears are listed as “motor gear”. Use some caution here; a gear with fewer than 12 teeth (at 20 degree pressure angle) will have an undercut tooth profile. Undercut can be avoided by using “addendum modification”, which is what I suppose Gizmo’s Zone has done. When a modified pinion is meshed with a standard gear, the pressure angle is more than 20 degrees (which is no problem, even though we may have used a 20 degree cutter) and the center distance is more than standard (which is also no problem, if you are able to calculate or fudge the correct center distance).