Start out with about 4 or more purl sts between the cables and decrease there. That ought to give you at least 3 decrease rounds. Also, with cables you don't cross on every row, but every 4th, 6th or 8th one, depending on the pattern. So you could work it so the last cable cross would be when you have 1 purl st between them, then the very top of the hat would be in rib or stockinette where you could do k2tog.
sue- knitting heretic