The dimensions of the P90 will mean that it will not fit in a violin or viola case. A cello case maybe with room for extras if you remove the inner lining and maybe laser cut some foam for the mags and stuff.
I just bought a $20 gun case from Canadian Tire. Does it's job well. I'm thinking if you really want to use subterfuge in transporting stuff like that. Then stuff it into an electric keyboard case (and with a carrying sleeve if you have one).
ie. This: http://www.drumza.com/images/SKB4214WKeyboardCase.jpg
plus this: http://www.uniquesquared.com/eBayImg...case_thumb.jpg
The keyboard case will definitely have enough room and you can fit your mags and extras in it as well. Not only that, with the outer shell/carrying bag people will just assume you're off to jam somewhere or something.