Lol @ sleeping bag. Take it out and sleep in it. Unless your kick is like 40x40, a sleeping bag is way too much muffling.
If you just absolutely insist on muffling your kick, a small pillow that just barely and gently touches the batter and/or reso head should be plenty. Those cheap, undersized 5 dollar pillows at wal-mart work perfectly. People routinely way over-muffle their kicks with whole blankets, sleeping bags, etc, and then wonder why they have no power or body.