Hang it underneath!
You'll need an AGM or Gel battery, but it's worth the extra cost to be able to mount it sideways.
I've had an AGM hung between my frame-rails for years (in a vented, drainable, steel box), fairly close to the starter battery and super-low and centered weight.
Unless you drive through lots of salt-water, an occasional dunk won't hurt a battery. River water is seldom salty enough to be a good conductor, bot terminals can get wet and not cause problems.
Only other consideration is that the cable connecting the two batteries (and the ACR if you use one) is fused at both ends. I use an watertight Blue Sea breaker on the box that secures the battery to the frame, and one at the starter battery. Power can potentially flow both ways, so both power sources should have protection from damage/shorts in that cable.
A lithium underneath may get too cold to charge happily with some sort of heating below 32ºf, but otherwise it will be fine there too. Just make sure you have a regulated DC-DC charger to keep the battery from destroying your alternator.
