I am a new list member & I'm glad I finally have something to contribute.
Yakima roof racks are excellent & do have tower rack "feet".  Another
equally good manufacturer is Thule (pronounced "toolie"), a Swedish
company.  They can also be found at bike & ski shops.   Thule has a super
tower foot that, I believe, is 11" tall.  This is approx. 3" taller that
what Yakima offers.  Both Yakima & Thule offer kayak mounts that are called
cradles, padded semicircular mounts that can be adjusted to the width of
the kayak.  Their other carrier is called a stacker, which lets you stack
multiple kayaks on their sides between vertical bars.

Mike Peters
'93 EV MV (Mjk Bus)
'87 Volvo 760 Turbo Wagon (for Sale)