Dani: Sanc-tus Wen-ces-la-us Rex -- 7 syllables, just what you need (cf. Good King Wen-ces-las looked out).

As to the comment in the Oxford Book of Carols, whoever wrote it is entitled to his opinion, but I think he overdoes his expression of it & may reasonably be suspected of wishing to spoil other people's fun. Its popularity with *me*, at any rate, is not due solely to the tune.