Get a double-bed size poly-cotton bedsheet, old or new, scrunch it up and stuff it inside the drum. That's it!
Why this method in particular? Because you can adjust the damping to suit the room just by tilting the drum backwards or forwards and giving it a quick shake. You can have the sheet sitting mostly or all on the bottom of the shell, or touching the front and/or back head in any combination and to any degree you choose. In reality, of course, you don't worry about what's actually happening to the sheet inside the drum - you just tilt and wiggle it until it sounds right, then stop.