I somehow thought that might be the case!
Guess i'll need to fabricate something
The dukes of Hazzard horns are just normal air horns from halfords - kept being cut up and found that the zed just didnt seem to cut the mustard when it came to the horns!
http://www.halfords.com/motoring/car-parts/electrical-lighting/car-alarms-car-horns/ring-twin-car-air-horn
They are now very very loud.. so i suspect a single cover isn't going to make a huge amount of difference :D