The battery compartment makes the lower hull rigid, if you remove the whole thing then a quick and cheap way to restore the rigidity is to cut a sheet of 2 or 3mm styrene sheet to fit it there, and hot glue it in place, then remount the board, battery etc on that. Again, leave a hole so that you can get to the battery plug from underneath.
The same goes for the magnet route, when the upper and lower are screwed together the hull becomes a very stiff box section, if you use magnets it will require some styrene bracing in the lower hull to stop it flexing.
