I’m on a roll!
Here is the core and the knitted jacket, ready for assembly:
And the wool core in it’s little jacket (I love this photo for some weird reason):
And the photo below shows how much excess fabric there was before felting. I pinched all the excess into a ridge so it would show up:
And here they are after felting:
It did take two trips through the washer and dryer before the jackets felted down around the core. I think it would have been fine to use only 8 stitches for the cast-on, instead of 10, for the size core I was using (8.5 to 9″ in circumference). If I ever make more, that’s probably what I’ll try.