Note in my proposed design I'm planning to raise the bottom woofer about 8-12 inches off a base plinth to facilitate future use for a passive crossover and/or use that space for sand or shot fill. So there will be an additional "bottom" shelf there that I will probably not cut through.

Dampening with 1/4 masonite or mdf sandwiched with 3 or four layers of 30lb roofing felt screwed, not glued to the cabient. Fibergalss insulation also works well, but consider the volume when adding fill. I'd make access panels so you can adjust fill as a tuning element.

Just my experience.