The synthetic hollow stock on my shotgun is full of lead.
I first epoxied a couple of lead strips around the entrance to the stock, just in front of the recoil pad.
That provided near perfect balance, but not quite enough recoil reduction.
So I added a couple of hunks of lead from an old defunct swimming pool cleaning robot.
It's now a bit out of balance to the rear, but shoots much nicer.
Some folks use lead shot in bags.
You could always fill it with gold coins and never be poor.