He ain't heavy...he's a SNAGGER!!...
I considered the same thing...but like you, what to use?...I considered sand, but it would need to be water-proofed some way...wrapped good in heavy plastic or something...I considered lining the front-most cavity in front of the bulkhead w/plastic and pouring full of sand, but that's gonna be TOUGH to get to...there might be a spray in liner (undercoating?) to coat the inside before pouring in the sand...
TEST...in the water, sit 5 gal buckets of sand (or sand-bags)up front to see how much it'll take to get the thru-hulls outta the water
It wouldn't be outta the question to relocate the two rear batteries up where the trolling motor batteries are...