Sometimes you can fill those plastic stock butts with insulation foam in a can you buy at Home Depot.That dampens out that hollow sound quite a bit.
If there is channel under the barrel you can lay part of a carbon filament arrow in the and fill around it with Brownells Accruglass jell.I have mixed the straight Accuglass with poly beads and filled that channel .I think the arrow gives a stiffer foreend though
