Tony, provided the holes in the extrusion (once the fittings are removed) are perfectly round and there is no sign of cracking or tearing, you should be able to simply plug each one with a blind rivet of the correct size with no real reduction in structural integrity of the mast. Don't forget to use good quality aluminium rivets as they are only being used to plug the holes. It would still do no harm to use an insulating paste though.