One of these will work for one footstrap (you will need 4 for a typical windsurfing board with four footstraps).
Foot Strap Anti-Twist Washers
The Dakine Twist Control System gives your footstraps a more solid connection and makes them less likely to twist when you are getting your foot in or out of the foot straps. Each kit consists of two stainless washers and two sets of plastic caps (one set to use for a single screw attachment, and one set to use for a two screw attachment) and will set you up for one footstrap. So, for a typical board with four foot straps, you would need 4 of the twist control systems. This does not come with footstrap screws.