Here's how I did mine...
they're fairly snug in there to begin with, so all I had to do was put a hook-eyeinto the transom behind the batterys and a short bungee from that to the front of the box...easy in/out...easy's good...
Oh and the 2x4 spacers under one side of the bottom of each box...screwed to the box, not to the boat...slides right out...
And the most important thing is security...notice the Norton Snaggers over each battery...keeps out unwanted intruders when the dog's not around to lick 'em to death!! :o...