[Argo]

Here's another idea to toss into the pot ... If the rubber straps are too big to fit without shaving, you might want to consider checking out a marine supply store (Westmarine, BoatsUS) for some heavy duty nylon battery hold-down straps. Heavy nylon strapping, very strong, tighten it most of the way by pulling strap through center clamp like a belt through a buckle then pull clamp back to tighten and fasten (like the snap clamps on the air box except lots heavier and stronger). I've been using them to hold in a couple of large marine batteries on a sailboat for years and, even though the boat spends lots of time on its side in heavy seas, they have never stretched or loosened in the slightest. The only question is how acid resistant they'd be if the battery leaked.
The ones I have were sold seperately, don't get the ones that come with the battery boxes as they're just cheap cr@p. You might be able to find the heavy duty ones in an auto parts store; marine stuff generally is more robust, tho, because of the high cyclical stress loads that boats encounter.

Argo
88 Targa

I just read Warren's post - the polypropylene straps that come with the marine box are the ones you should avoid. They're strong when new but age poorly, wear easily and will split and fail when you need them the most (BTDT and it wasn't pretty!).