John,
I haven't tried this but have wondered if polystyrene concrete could be used to damp the gantry without it getting too heavy. I found an article once (don't have it to hand I'm sorry) that described using up to 60% by volume of polystyrene beads (you can buy them for making bean-bag furniture) to make concrete for light-weight structures. It's still quite strong at that percentage and I think the density came out at a little over 1Kg/litre.