Why not just nicely cut a 4" by 6" square of sheet rock to serve as a removable recessed panel. Trace the panel onto the ceiling. Saw the ceiling for a nice fit. Coat the raw edges of sheetrock with a fixative to prevent them from shedding dust. (Perhaps reinforce the ceiling opening per bmitch. Around the 4x6 cutout with some 1x2 whitewood or similar glue or screwed to the backside of the ceiling along the 6" edge)
Instead of mudding the patch in place, rig it up as a removable panel: glue two 1" by 7" strips of quarter inch plywood (or similar in popsicle sticks) to the back of the patch in the 6" direction. The strips will overhang the panel by 1/2".
The panel will insert and hang and be easy to push up and slide off to the side when access to the valve is needed.
or used any other material for the panel. 4x8x1/16 inch sheet aluminum bent twice at 90 on each edge to form a lip, or 1/8" plywood.