Another option would be a simple piece of 2x3 behind the box, fixed into the concrete. Certainly I wouldn't try fixing the box into 12mm ply: The stuff is far to thin for a screw into the edge, and even if you managed it it'd tear right off if pushed. You'd also have problems getting conduit to the side entries on the box.
You could screw a 2x1" batten to the back of the ply either side of the cutout to give you plenty to fix your flush box to
Theres no reason why you cant use dry lining boxes, or if you want to be really posh, use metal dry lining boxes