Protecting wood from being too close to warm metal seemed pretty important, and a bit of protective sheet metal was all that was needed to do the job. The water heater closest outside is a safer place these days because all the wood near the exhaust flue is covered.
And if you're going to buy enough sheet metal for one job, you might as well buy enough for two jobs. So I went ahead and protected one of the beams in the attic from the exhaust flue near it. It's an exhaust flue that's not currently hooked up (it's left over from an old furnace), but it was on the inspector's report so I figured I'd knock it out while I had some sheet metal laying around.