I have heard that some have success mounting new horns in the pods on the IS. Mine is a 2001 Interstate.
I've put a Stebel Nautilus in the right side pod, even though there's a voltmeter in the same pod. I imagine it would be difficult if you have the pod lights as well, though. You still have to point it downward. The pod doesn't protect it quite enough if you get in a real hard rain. I had the gargling frog sound come out of mine when I first put it on pointing upwards.
As for mounting, I just used a homemade bracket, and bolted it up to one of the radiator bolts. The radiator is rubber mounted, so make sure you ground it separately. Might take a few tries to get it to sit properly, and let the pod fit around it, but sounds great after you get it.
I want to put another in the left pod, just for added volume, and symmetry. The sound definitely is louder on the right side, and I want to blast people on each side if necessary.