As David said, some 1" battens.
I'd just go for 1"x2" strips, abrade the hull and epoxy
them down, them paint
or similar paint
to ensure no water
Then you can screw any kind of hardware
, straps, etc. to the battens to hold the boxes down. If you can grab the straps, put your back into it and heave, and you think that would tear them out? Use stronger fittings. Don't underestimate the force that a battery
can exert if the boat
is bouncing around, or hits something.