One of the things I always worry about with the Ricoh Theta S is damaging its lenses. Here are some lens caps and protective covers for the Theta.
1. Plastic lens cap
One vendor describes it as "shock resistant, scratch-resistant and anti-bump for extra protection." It slides on and off the Theta. The cheapest one I've seen so far is this one (as of the time of this writing):
Here's another one that costs a few dollars more but includes a lens cleaning pen and cleaning cloth.
2. Silicone cover and lens cap
This one includes a silicone cover for the Theta as well as a separate silicone lens cap, and includes a lens cleaning pen and cleaning cloth.
Here's the black version (there's also a bright yellow and bright blue version):
If you prefer, there's another kit that includes a wrist strap and connector instead of a lens pen and cloth.
3. Myking lens cap
This is a thick solid plastic cover for the Ricoh Theta. Looks really sturdy.