Megaphones are usually louder than straight pipes....remember the old cheerleader megaphones. I just ran a track with a 104db limit and megaphones were not working. Go to a dirt track shop and get 2 Schoenfield 8 inch header mufflers(#83030 $15.95 each) and make up 2 pipes with these and a turn out. Works well and will not hurt performance. Make sure toi direct the turn out away from the meter.
http://www.schoenfeldheaders.com/mufflers.htm