That’s a very smart way to go about this , you drill the end of the 2x4 as you say any length you want easy to split later and retrieve your lead.Use the drill bit in a 2x4 how every long you want the weight. Pour lead in it let cool and bust out of 2x4 with chisel ; then clean up as needed.
Test fit with a little room add a little caulk to both ends for a cushion reattach butt plate.
If they are hex shank... Shouldn't be a problem. Forstner usually come in a straight shank. If it's straight, make an extension. Drill/ream one side and tap for a set screw to lock. Turn other end if need be to fit drill. Many other ways too...shrink fit, silver solder, pin it. Get creative, lolYou can get those extension at about any hardware store. I’ve used copper tube to bore a hole in the foam as well. Just cut some serrations on the end of it.
Wallow it out then install a pvc liner
Hey, he even spelled it right. Waller is mostly used around here...lolWallow? That's akin to hog it out?