My Sportster has that egg crate open cell foam as noise deadening in various places, and some of that is hung onto the styrofoam floatation blocks that were haphazardly glued with permanent adhesive to the inside of the hull so now they're all lose of course but still there.
There should be room for another battery somewhere though?
I guess if I was installing a second battery in my boat I would consider placing it under the helm or if I wanted to install in engine hatch I'd construct a battery tray mounting block by epoxying a dried wooden block in there to fasten to. Simply coat the wooden block in resin and fiberglass cloth to make it waterproof then paste it into place using epoxy putty.
You'll want to obtain a good size piece of glass cloth to wrap the block and about a quart of epoxy resin, mix the resin with activator (usually 2:1 ratio) and wrap that block of DRY wood in cloth like a Christmas package and wet onto block with activated resin and set aside for curing.
To make epoxy putty, simply mix a good amount of saw dust adding the dust while stirring with spoon into mixed activated resin to make a peanut butter consistency paste for cementing the wooden block in place.
Make sure to clean away oil and grease using acetone, and dry the area with hair dryer as necessary, before pasting the glassed block into place.
Some things you'll need are:
Quart of epoxy resin, activator, glass cloth, scissors, old plastic credit card to spread activated resin into the cloth, old kitchen spoon for working and mixing resin. Vinyl gloves, quart of acetone, maybe sandpaper.
I simply enjoy everyone else's stereo, especially when they blast the stripper bar pole dancer music.