If you're going to use bottles - DON'T use a soda bottle or milk jug. Use a heftier container, like a bleach bottle (already a nice white) or the jugs that windshield washing fluid comes in. And remember to change them frequently, as exposure to the sun will quickly degrade them.
Maybe filling it with the NONEXPANDABLE "foam-in-a-can" would give it a little more heft?
But I've gotta agree with "centralmichigansailor" - If you're concerned about turtling, Hobie Bobs [IMHO] are worth the time it takes to save the extra $$$ to get one. And it will look a WHOLE lot better.
joanie / Hobie enthusiast: 4 H-14Ts, 2 Waves, FloatCat 75, 4 Adventure Islands, and my DUNE Tandem Island!
. . . . . . . . . . . . . . . . . . What can I say? I never met a Hobie I didn't like . . .