you won't find heavier than 30# felt nowadays, used to be a 60# we called 'bulletproof paper" [replaced with Tyvek], but even that wouldn't be thick enough, just denser
how about some actual felt, like carpet underlay, which comes in various thickness, bit of contact cement to stick it, might find a flooring store to give you some scraps - might check the weatherstripping section of harware or builder supply places too, I've seen 1/4-1/2" felt on a roll for door/window bottoms
Bill
