I think we have been through this a few times. Remember you need at least 3 cubic feet (1.5 in each hull) Here are a few options:

1) Easy and expensive 2 floatation bags made for monohulls $80. Call Layline or a Vanguard dealer can help you.

2) Pour Foam - mix 2 liquids and it turns into Styrofoam. Pour through the portholes into a trash bag. You need ~3 ft3 ~$80

3) Styrofoam - cut a sheet into strips that fit through your portholes a 4'x8'x 4" sheet is around ~$20. Look in the phone book and call for best prices


4) Foam noodles - They are on big sale now

5) Super Heavy Duty Beach Toys - They are on big sale now

6) Empty 2l soda bottles

7) Be inventive, go to the discount stores and look in the sale bins. I have heard some good ones over the years. Really cheap life jackets, Foam pool lounge, Giant green foam frog, Puncture proof tractor tire and my all time favorite "The inflatable pool woman". You were supposed to impress your friends by having this strange looking silicone rubber woman floating in your pool. She normally floated face down. I saw 4 of them and a piece of plywood being used as a work raft