I think you could probably use any cover/fitted pattern with a shape that you like and just cut out the wool in that shape. Maybe get a freebee fitted diaper pattern and try your hand at a few of those before cutting into your felted sweaters? The good thing about felted sweaters is that they won't fray, so you could do without serging or turning and top-stitching if you wanted to.
I've been out of the diaper sewing scene for a LONG time (10 yrs!) so I don't really know what's out there for free patterns that are popular. Maybe someone else has recommendations??? I'm just starting to get my feet wet again so I would be interested in what's out there too.