They are in fact the Sport-a Seats mentioned above. The striped covers are actually slip covers I made from beach towels (wash and wear easy!). These seats are fantastic and you can buy little lumbar pillows that attach to them. We've had ours for years. The bottom of our bench cushions are made of that rubberized mesh that allow them to drain / dry if wet and I would say that adds some traction to prevent them from blowing away. We put them below if we are dealing with bad conditions, etc. With a bimini and dodger and the fact that top speed is about 6-7 knots, winds inside the cockpit strong enough to lift the cushions has not been an issue.
Hope that helps!
Hope that helps!