I'm not an expert in this in anyway, but for a first try I would form the full mast in styrofoam (32kg/m^3) and then vacuum a layup over this and leave it there. Should only add 2.5 to 3 kg to the whole mast and styrofoam is easy to shape and I think it it just strong enough to withstand the compression under vacuum. It is cheap, doable and has reasonably succes expectancy.

Wouter