Ahoy, Chuckd,
I got a roll of lead sheet from a roofing supply house. Called 'two pound lead sheet' for 2 lb/sq/ft. Cost was not bad.
My genset box is a 'sandwich': 1/2" birch plywood, two layers of 30lb roofing felt, one layer of two lb lead, two layers of roofing felt, one layer of 1/2" birch plywood. VERY effective.
Enjoy /s/ Bob