Ukuleles are small and light, but adding a strap can be tricky, especially at the back. This design, inspired by MyMiniFactory's instrument spare parts contest, includes a hook for the headstock and another for the sound hole, to allow a strap to be easily added and removed.
This style of hook is a fairly traditional approach, but somehow I'd never actually got myself one. Thanks to 3D printing, I've now remedied that!
Specifically, I've used this on a Kala KA-SSTU-C Concert Travel Ukulele, but it should work just fine on any uke, provided the headstock and sound hole thicknesses aren't too different.
Oh, and I should also point out that while ukuleles are very light, you will want to think carefully before trusting your beloved instrument to some 3D printed plastic! That said, it's not really any different to the danger of a guitar strap coming off, and that's not exactly unknown.