Cheapest and fastest route I can see would be to cut some lengths from a 2x4 and secure them to your existing racks with stainless steel hose clamps, or some U bolts. You could make the racks 8 feet wide if you wanted, but it might be difficult to pass a semi .

Bow and stern lines secured to the car at the bumpers or under the bumpers will keep the boats secure even on racks which are closely spaced. Many times there are rather random holes in the frame of a car. If you can find one in a convenient location, and get an eyebolt through it, you'll have a secure point to tie a rope to.

Try something simple like this and do some slow-speed test drives around the neighborhood until you have confidence in the setup.

Hope this helps.

PGJ