A customer has a coffee cart which needs to plug into a 50A 120/240V 3 pole 4 wire CS6369 recep
. I want to install the recep in (as opposed to surface mount) an interior finished wall. The building is wood framed and has a dropped ceiling. I will be passing through fire walls, so my plan was to run conduit from the panel, and then terminate to flex where it will go down the wall. I want the installation to not be an eyesore if the cart is not at the location, so the recep should probably be covered by either a weather proof cover or a standard cover like HERE (top three pictures)
. However, these covers mount to an FS or FD box. My plan was to remove enough sheetrock at the recep location to install a 4S deep box w/ brackets. If I knew the coffee cart would be plugged in 100% of the time, with no chance of the customer deciding selling coffee isn't working out, I'd simply install a box cover, like the HBL50SC shown at the bottom of the above page. However, I'm not sure what to do from there. I can't use a standard single gang mud ring, because it won't have the four corner screw holes required for the cover. Does anybody have any suggestions for a clean installation?