I do a papier mache mask project with the kids every year and we use newspaper stuffed plastic grocery bags and they work great. You stuff the bag as full as you like,tie it and then tape the bag with masking tape to the size and shape you desire. Tape the bag to cardboard and then papier mache over it. It works great. With the mask we only do one side, but I see no reason why you couldn't do the other side later, leave a hole at the end to pull out the stuffing and that would be where the tail would go if you are making fish. With Jelly fish I think it would be a perfect solution.
Tina in San Diego
PS. - I have this year rejoined the list - I used to get the messages individually which was a pain. Now I get them in a digest for one day which is easier, yet I am confused as to the easiest way to reply to the messages. I have been composing each message to the getty site and just adding the message name. Is this the best way or is their another?