We have pins and snappies. It falls down either way
the snappi has the added fun that he can reach in and take it off, and usually sticks it in his mouth. I knit/crochet all our wool covers but can't justify the time investment to make more if I can't guarantee it'll work, kwim? I tried the sweater method once; the one disintegrated at the felting stage and then my dog ate the other one and I haven't had the heart to try again