3 layers Plaster wall & Technical Hatch

Hi people, We've been working on the construction of our new studios for the last 6 months and I have a construction detail I have to go thru since my acoustician went on holidays for a few weeks. In the back wall of my Control Room which is made of 3 layers of plaster board ( 1x 18mm / 2x 12,5mm from the outside to the inside) mounte on steel studs, I need to keep an access to a drain. I hope we won't need to access to this drain anytime but I have to keep this accessible in case... This doesn"t need to be large, only 60cm x 60cm but I only have a rough idea of how to design this properly. Would anyone have a sketch or a quick draw of one of this piece of construction? I think I have to start with the same idea than the overlap of plaster layers in the corners, am I wrong? Thanks a lot cheers benoit

build it like you would a door, with frames and seals.