Nothing wrong with flat white.. I dunno which would be more convenient for those doors, painted cardboard, or panda film.. Probably the plastic..
Honestly though, I don't think you can get away with it if you intend to have visitors in that room for your pc.. There is a catch-22 between noise and odour control needed for a setup like that, and even if ppl don't suspect, if they hear something you'll get put on the spot with innocent questions.. Your only chance would be to launder the noise.. Replace your pc fans with whiney ones with messed up bearings, run a loud exhaust fan visibly and claim its for smoke, do whatever you can to to make the room annoyingly loud as long as you can rationalize it..
Still though, the larger the DMZ you can keep between your grow op, and your visitors the better..