Dampening material

Whats the best material to damp the inside of amplifier covers?
Dynamat -cut it your dimensions taking care to cut out the ventilation openings.
Herbie's Audio Lab's Grungebuster Damping Sheets
Stillpoints's ERS 8in x 11in Sheets

You can get them from The Cable Company.
If you are serious, then using 'lead' wallpaper is a must. Yes, such a product does exist. Years ago I used it on a wall to great effect. A must is to wash up carefully after handling it.
Why not go topless?