Why don't you mold the case so that it can not be put in backwards?

By the way; several people have suggested this, and I'd like to put in a
"NO" vote.  There is enough variation in the physical parameters of
batteries that this may prevent your device from working properly even if
the batteries are inserted properly.  I've got a couple commercial products
that are very annoying in that they get flakey with some brands of batteries
(which don't have sharp enough "nubs" on the plus side), and don't work at
all with certain improvised battery arrangements (ie NiCd or NiMH cells
rescued from a damaged pack, which tend to be flat on both ends.)

BillW