This item would be a definite TWO star product, if my husband had not read a review of it elsewhere.
The trick is marbles. You'll have to buy maybe 4 or more bags of them at your local dollar store.
Pop the marbles in through the "fill" hole in the bottom (an interesting excercise in agility if you're actually sitting IN the spa) and you've got enough weight so the cushion won't float away or out from under you.
Thanks to whoever wrote that original review.