A co-worker and I sat for a while one day and tried to figure out how we could make an expanding mitten, one that would get bigger as the different animals snuggled in - we couldn't come up with any good ideas, so here is what I made instead.
A mitten out of card stock, with a pocket in the back that I could slide the animals into as I told the story.
If you go to Jan Brett's web-site: http://www.janbrett.com/, specific page - http://janbrett.com/tell_your_own_mitten_story_animals.htm you can print off a picture (black & white outline) of the animals from the story. I colored and laminated them to use for this flannel board. I also used them for an arts & crafts after storytime once. I cut mittens out of construction paper, punched holes all around the edges and let the kids use yarn to sew their mitten together, then they each got a print out of the sheet I linked above so they could fill their mitten with all the animals too.