Designing tuners was probably the most difficult task in this project. I looked at several designs even buying some from ebay.
Commercial tuners are designed to fit on a head stock and are designed so that where you hold the tuner (the knob) and where the string is attached (hte post) are in different places. Because the Sq'Uke is headless, I wanted the knob and the post to be close.
I first tried eye-bolts, a design that I gleaned from a CGB community, I was concerned about the holding power and unscrewing the nuts. I was going to use nyloc nuts, but my experience with them is that they lost their holding power after a couple of turns. Besides, I couldn't control how tight they locked. The breakthrough came when I made my own locknuts by hammering them oval rather than round.
I used the eye-bolt idea on a couple of Sq'Ukes, but they took up so much space. The next idea was to use normal machine screws and drill a hole. Several drill bits later, I came upon the idea of cutting a slot.
And that is the design to date.
Tuners are used to change the pitch of the string by increasing or decreasing tension.