More and more people wanted sidearms and the easiest casing to use that I could get ahold of was this lightgun casing used by the playstation.

It is big enough to build a 1 ah rechargeable pack in there as well as a 50mm speaker and all the circuitry.
I use the lens already built in there which while not great gives about 50 metres effective range, more than enough for a back-up sidearm.