The other place that would be awesome, but a bit more work, is underneath the transmission. If you look on the right side, there is an unused hole. An AGM battery would snuggle in behind the oil filter cover, and there are a couple that would sit higher than the lowest part of the engine there.
Unfortunately, there is no matching hole on the left side, and there would be heat and vibration issues. But it would definitely be gone and out of the way...