Great question, especially since Thule has updated their foot pack models over the last 10 years. The specific foot pack that you need is dependent on the roof options of your vehicle. For example, a vehicle with a "flush" rail would need a different foot pack than a vehicle with a "raised" rail. I would need more specific details on your vehicle, in order to make the best foot pack recommendation (year, make & model, roof details, etc).
With that being said, REI's Thule Rack Fit Guide will allow you to plug in your vehicle's info, and the guide will help narrow down all the Thule rack options (including compatible foot packs). Fortunately, the Thule Hullavator Pro Kayak Carrier is compatible with all of Thule's rack systems.
For an even more personalized gear service, REI's Virtual Outfitting can directly help in a "Car Rack Outfitting Appointment". The free Virtual Outfitting appointment can be made over web cam or the phone, allowing you to ask an expert all your car rack questions in real-time.
Thank you for your question. Feel free to reply back with additional information on your vehicle, or if you have any additional questions!