Ok to install NEMA 14-50R in plastic, new-work, single-gang box?
Box fill calculations say you need a volume of 5 in³ per AWG 6 conductor that lands in, or is spliced in the box, plus an allowance of 2 wire volumes (5 in³ each) for the receptacle itself.  For a 14-50R receptacle, you will have 4 conductors, plus 2 for the receptacle, means that 30 in³ is required.

A 2g remodel box is only 25 in³, which is not legally large enough, but is exactly what we use for every Tesla charger we install.  It's a bitch to squeeze the wires in and get the receptacle screwed down, so I recommend installing the whole assembly before mounting the box in the wall so you don't break the drywall with the force of the screwdriver pressing against the screw head.

I'm starting to reconsider this practice and may start using a 4"x2-1/8" square box, which is the minimum actually approved for this application...