A digital tilt sensor would work good along with a siren hidden inside the passenger compartment. If you toss the shock sensor, I would suggest considering either a glass break, microwave, and/or infrasonic sensor. I used the Autopage Sis-12 infrasonic sensor in my Civic and have yet to receive a false trigger off of it!
I always toss the shock sensor that comes with the alarm in the trash because I have NEVER had luck with a shock sensor, no matter who installed it! For example, my Ford F-150 was hit so hard in the parking lot at work that the Acura TSX bumper was still attached to my trailer hitch and the alarm DID NOT go off! One clap of thunder, would set if off EVERY time!
ETA: I see that DEI has an Ultrasonic sensor that works based off of the cabin pressure too. Check out the DEI 509U because I highly recommend it, unless you have a convertible. Basically, anything causing the cabin pressure to drop will trigger that bad boy... pop a window, open a door, pop the sunroof, open the trunk, etc.