Hi Rangeball ,
you can fill a cardboard box with triple expanding insulation foam for about $4-8 . Look for it in the paint dept. at Wally World , make sure it' s the triple expanding kind . If pass through is a concern put some layers of newspaper in the middle before applying the foam . This is what I use for a gun target , and it will stop a muzzleloader round if you do it right . It helps to hang a piece of carpet behind it when you' re shooting .