Acrylic rod - cool idea, to transmit the light.
Trying to make a lens or rod look professional would require some expertise/experience, I'm afraid.
I'm leaning toward a 4" deep enclosure with a screw cover, and a cutout for the power module. I have the flush mount Eaton plate already, so an exact cutout wouldn't be needed. Mounting the enclosure with the plate perfectly flush might be tricky, but not impossible.
The only drawback - and it might be a deal breaker - would be that the Eaton flush mount plate would be flush with the drywall, but the enclosure cover would need to be slightly recessed.
Maybe with a clean cut of the cover, I could mount the plate to the inside of the cover, allowing me to mount the cover proud (sticking out in front of) the drywall.
I could probably have a small square cutout of the steel cover done professionally, for pretty cheap. I could probably figure out a way to cut an ABS or other plastic cover myself. Hmm, always looking for a use for my Dremel....