Cozy by LilyGrace
Here's a link for how to make a cozy sack. I made one of these for my new piggie and he loves it. I used some leftover denim for the outside and baseball print fleece (for my son the baseball fanatic) for the inside.
(Hope it's ok to link to another site on this one. These are not my photos or directions, just some that I found when doing a google search)