While I have a Thule bar set and towers that fit my new vehicle (2014 Hyundai Santa Fe), the instructions indicate that the bars should be set ridiculously close together.  So, I went with a single bar (roughly over the driver door) and a   (You need to Login or Register (mine came already assembled but was more expensive) which mounts to a trailer hitch tongue.  I modified the top t-bar to accept  a metal handrail post that would insert into the end of the bar to add side "wings."  This was then fastened to the t-bar by drilling multiple holes in the handrail post and a single set thru each end of the t-bar and finally held in place by   (You need to Login or Register. I use the wire loops on the pins as anchor points for tie-out (ratchet) lines down to the bumper to add lateral stability.  Before the tie-out lines attach, the top of the "T" will swivel, which makes loading a single canoe much easier.

Now the downside of this arrangement is that it blocks a pick-up tailgate or a lift-gate on a hatchback, so you will probably want to load your "back end" before placing the canoe(s) on top.

dd

Follow-up:  The "wings" (one each side) were crafted from these   (You need to Login or Register sections, cut off to an appropriate length ... holes drilled about every 4 inches so the remaining length could be adjusted and locked into place with the pins.  I also threaded an eye-bolt through the holes already present near the top as a tied-down point near the ends of the assembly.  I placed an adhesive rubber strip on the "top" of each of the sections and reflective tape to the rear-facing edges of many of the parts.