STC of a heavy single layer wall

I am building a vocal booth and would like to know: I intend on building inside out walls (Like in johns vocal booth design). I want walls to be modular (removable) so I will not seal with caulk. I do not need total isolation. Only partial so that singers aren't as loud. I was wondering if I create a wall that has inside out walls, would 1 layer of wall be enough to reduce the sound of voice? I was thinking of 2 layers of firecode drywall + sonopan (paperboard) with greenglue between the drywall. Also, what type of prehung door do you recommend that has a small window in it? I know it will be expensive... Also, my vocal booth will be about 5 feet by 4 feet. I do not want to build a frame for the ceiling. Would 2 layers of green glued MDF be able to stand on the wall frames? Thanks