I've accomplished this with various methods, but the easiest way I've found
is to use a vise.
The glued cases are generally 2 halves with an overlapping seam that glued
with instant glue. By applying just enough force to just the seam area, the
glue usually lets go with a sharp "pop" sound.

The flat jaws of a vise aren't ideal for concentrating the pressure to just
the seam area, but if some round metal rod (about 1/8" diameter) is held to
that location with tape, the pressure can be applied right where you want
it. The round rod is less likely than a sharp edge, to mark the plastic
case/box.

Any spots that don't pop open can usually be persuaded with a plastic tool.

I've opened wall warts, battery packs and other glued cases with this method
without causing any obvious cosmetic damage.

Cheers
WB