H.S.
I saw this in a magazine (parents or parenting): they made a "zoo" out of some bookshelves or those wood storage cubes you can get at Target/Lowes, etc. They used elastic spaced about 2 inches apart and stretched from top to bottom of the shelf. It was attached with screws (make small hole in shelf, fold elastic over on itself a tiny bit, there was some kind of washer over the elastic). The 2 inches of space and elastic easily let you cram in lots of animals and they were held in place.