Simple spring that won't move around like a metal coil spring. Use the one appropriate to the filament you use. Turn the spring on its side to print  for a better flex.  Start with version 1, if it is too stiff, try the next version. Print with supports at 100 percent infill. I printed them at 0.2 layer height in PLA. They are not bendy enough, so I am guessing a flexible filament would work.

Print Settings

 Rafts:

No

Supports: 

Yes unless you have excellent bridging.

Resolution:

0.2

Infill: 

100 %