Simple stretchlet with very large radii, which should make it last a long time due to reduced stress. Designed for my 2-year-old daughter's wrist but it stretches quite a bit in nylon.

Printed mine on an Ultimaker 2 using Taulman 645 nylon. I have had good success printing on 3M blue painter's tape with a cold bed and a head temperature of 245C, single layer (continuous Z aka "Joris"), layer thickness 0.1mm. The nylon is extremely flexible, especially when it's this thin and with all these large radii, so it can stretch quite a bit. Haven't tried this particular one in PLA, I imagine it would work fine but just be more stiff.

The twisted version has a 10-degree twist. I printed it at 0.2mm layer height. This one was much harder to remove from its base for some reason, not sure if it's due to the twist or the layer thickness difference. For the non-twisted version I was able to remove the base just by peeling it off carefully.