Felt crafts are great for Chipmunks, and they can make these while the Otters are making Otter Masks. You can pre-cut the felt shaped according to the templates below and have the kids glue them together. If you want them to be sturdier, you could also bring a needle and thread to stitch the elastic to the felt.
Liz with the 22nd Wildwood in Oregon created this Chipmunk Mask Template. You will need a few different colors of felt (brown, tan or white, pink for the ears, and black for the nose), glue, and elastic.