If I can’t use two halliards to ascend the mast I take two long loops of either tape or 8mm rope. A Prussik loop round the mast can be pushed up but holds under the tension of a fall. The second one is to attach above the spreaders so you never rely on just one attachment. Prussik loops were used in climbing before ascenders were invented, but they do work around the dimension of a mast. Also useful as a foot loop to stand in if you want to get above the height of the mast to work.
