[Naagzh]

Some type of bandwidth-reduction is a good place to start. Use an EQ to get rid of the low end AND the high end (IOW, filter out everything but the mids). Maybe a subtle boost around 4 kHz. Then, maybe add some distortion with Guitar Rig or Sansamp, or even send the signal out to a guitar amp (and then mic the amp).

Or, buy a megaphone. They're not THAT expensive, and fun for the whole family. I have a smaller toy-sized megaphone that sounds good, and, as a bonus, isn't very loud. While you're out shopping, pick up some cheap walkie-talkies, and a green-bullet harmonica mic. Ebay is wonderful for this stuff.