How about stuffing some large bags (the mesh bags someone else suggested would do) full of styrofoam packing peanuts -- I save them from mail order shipments -- and duct-taping the bags inside the ends? Test it in the water, and if the peanut-filled bags provide enough flotation, you can then make a better substitute for the duct tape attachment.

Not pretty but it's cheap.

Or...another ugly but cheap possibility: overinflate some mountain bike inner tubes (if inflated without the constraint of a tire around them they expand an amazing amount). Tape a few of those together and stuff 'em in the ends. If some biking friends have retired tubes with multiple patches (especially patches that overlap other patches), try it with those first.