I have an Air Zounds horn on my Bike Friday since my dog-induced accident in 2005. DH mounted it on my Salsa Poco bars up near the stem where there is no bar tape. Fit fine, but that was an older model. You don't mention what type of bars you have or whether you're trying to put it on a part of your bars with bar tape -- I can see how that would make the bar too wide for it.

It really does work! I rarely have to use it, but I have blasted a couple dogs that ran out on the road, and it normally stops them in their tracks long enough for me to make my getaway. Also good in case of a motor vehicle or another cyclist not seeing you, but it is indeed VERY loud. If you are riding with someone you don't normally ride with, warn them that you have it so they don't get a complete shock if you have to use it. And even if you've warned them once, try to warn them again right before you actually go to use it!