After talking with Steelblitz late last week, I thought I would give this a try. I disassembled the $18 Radioshack mini-amp. Mounted the speaker onto a 2" pipe that had been notched to fit down into the jet pack. The sound comes out throught the middle vent on my MLC arena pack.
Made a plastic platform to mount the amp. I had to glue in a little bit of pipe in the middle to raise the amp up. It won't sit flush with all the solder connections and such. I used CA glue on everything.
Cut the inside lip of the JP and a small notch on the backplate to access the on/off of the amp without removing the 4 screws. It's flush with the backplate and doesn't stick out. I had to open the left strap hole ever so slightly to get the mic plug through. Did the same to the Cruzer backplate, and you can't even see it. The only reason I would need to open the JP was to change the 9v battery.
As far as dressing. The mic has to go through the backplate before the strap. connection will be made outside the vest, between the vest and the backplate.
What do you think? The volume is decent, I've always been heard just fine through my helmet. This will help add a touch more volume.