They put them into the lights, too. A very professional job. Unfortunately they wouldn't do my Nitecore HA11 (AA powered) or any of my 18650 lights. They said they were trying to focus on their core competency. I had never heard of a service like this before! Highly recommended.
I haven't had trouble with the E3A O-ring but for such a minimal setup I'd consider a coin or button cell light, or maybe a 10180 light. Those can be much smaller than an E3A.
I guess see what the E3A page says about the finish these days.