Maybe two thin speakers, something like this:

http://cgi.ebay.com/ebaymotors/NEW-S...75590266QQrdZ1

OR

I might skip the mixer and the amp and just go with a bullhorn, mount it hidden in the fender somewhere, and then extend the hand unit so it reaches the cab of the crossfire, something like this:

http://cgi.ebay.com/50-Watt-Commerci...QQcmdZViewItem