The several design found on Thingiverse weren't up to my taste. I wanted something:

• 3d printed (with no support)
• Adjustable
• Not colliding with other pegs (remember: a ukulele is small)
• Not too ugly
• Compact

After several attempts with a traditional design, I've ended up by designing a key AND a peg that allow to apply sufficient force.
It's done to be compatible with the ukulele found here: http://www.thingiverse.com/thing:268090
Great sound by the way!
I've printed mine in ABS, but PLA works just as well.
If you find ABS too slippery, some talc powder makes wonder.
25 to 33% infill seems to be enough, but be sure to have 4 shells at least.

Once printed, insert in the ukulele and put the cord into first hole then directly into the next hole (a quarter turn) and so forth until you got it secured (it took me 4 passages to do so, YMMV).
To help you tune, you can also take advantage of the several iphone, android and even windows phone Ukulele tuner out there!
Happy playing...