"...taut bow and stern lines to the bumper areas are still required...."
Seems like it will be an interesting balancing act between too tight and not tight enough![/QUOTE]
A few thoughts...
I've carried two 17' sea kayaks (and canoes) for 10's of thousands of kilometers on various wagons and trucks, using padded crossbars with no saddles of any kind, and haven't come close to losing one yet. Here's what works for me:
Each boat is lashed to the front and rear crossbars (currently Thule bars on footpacks to fit my '99 Saturn SW1), using separate nylon cinch straps fore and aft on each boat (4 in total). These are tightened until hard (alternatively, you can use pieces of rope rather than straps, using a trucker's hitch to obtain sufficient purchase to get it tight).
If carrying two kayaks, I run the same noseline thru each boat's bow attachment point, tie it off at each bow, and then lead it thru webbing loops held in place under the furthest forward fender bolt on each side of the vehicle's bonnet. If expecting heavy winds, I'll use a second noseline per boat, crossing this one to the opposite corner to create a X and hold the hulls together. If there's one kayak or a canoe, I just tie the bowline's centre point to the boat, and lead the ends to and thru the loops, forming a V.
Besides providing security, the noselines act as an early warning system if anything starts shifting around up there...
A similar approach can be used in the rear, but it isn't nearly as critical, since you won't be generating any 100 km/hr headwinds in reverse. I usually don't use one unless I'm expecting to drive at highway speeds...
In either case, a trucker's hitch is used to apply
LIGHT pressure on the lines; there is no benefit to tightening the nose and stern lines until hard, as they are primarily a backup in case the rack or a strap should fail. Just snug, yes - hard, no. This is especially important with plastic kayaks; my Cape Horn 15 can bend as much as 4"-6" at the bow if the noseline is overly tight. I also find I prefer to carry the Cape Horn deck-down, as it distorts less due to the deck's curvature. My stitch-and-glue ply&epoxy VOLKSKAYAKs travel hull down, canoes upside down (strength, wind resistance, rain, etc.)
Finally, I check the straps and lines every time I stop; it's long since second nature to get out and check the four straps and the noseline to make sure it's all snug and tidy.
Oh, and, yeah, enjoy the Mirage - kayaking is about the closest you can get to being an aquatic creature without having to swim...well, you will swim, too, but that's another story...lol...