Foam base made of flexible and soft expanding foam

you can order pure cast or carved one, with cut mouth, eyes and interior
all bases will come pre-hollowed but will require additional carving

sometimes in casting foam happen air pockets, that´s just what foam sometimes do, I refill them with another casting foam or upholstery foam, it will have no effect to final look of fursuit, surface on some places just might look funny. You can cover fixed spots with piece of fabric before gluing fur for perfectly smooth surface under the fur

in last photo you can see an example of head made with using this base