I also own aThule #430 Tracker II Foot Pack and Thule #TK1 Tracker II Attachment Kit and plan to implement the solution(s) posted by WesH and DSalter. Many thanks for figuring this out!
I note that WesH recommends the following to keep the screws from seizing:
WesH wrote:
I sprayed some lithium spray grease on the screws in the slots to serve and a sealant to help retard rusting. And do that every once in a while just in case I ever want to remove them they will not be seized up.
However, I note that the original Thule screws have medium strength removable thread lock applied to them to add some extra security. Wouldn't it make sense to apply some medium strenght thread lock to the screws instead? You could always apply some rust inhibitor to the exposed threads later after the thread lock has set (the thread lock creates a seal between the threads and the nut in the absence of air).
Martin