Hi DC

Constrained layer damping sheets need an outer layer of thin steel bonded to them to make them work. I don't think you'll get much from just wrapping something in the butyl material.

They work well on large thin sheet materials such as air conditioning ducting panels etc., but you'll not get much if anything on small beams. If you do it will be at very high frequencies where the deflections are tiny, so of no practical benefit for what you need. Also by the time you flood this with the epoxy quartz that is where the benefits will come from.

I wonder if you'd be better off with a couple of 80x80 steel RHS beams across the width, filled internally with sand, then fill the whole area with sand and topped off with a 10mm ecocast aluminium plate tapped with a matrix of M8 holes . . .?