I prefer to mount my boats upside down as I worry about closing off the cockpits in a manner that will tolerate highway driving. Also I maybe the cheapest guy on the forum . . .

My vehicle came with a roof rack that is robust but the crossbars are the wrong shape for carrying boats so I made a pair of crossbars from a 2 x 4 which are mounted on edge. On the underside they are customized fit the fore-and-aft bars of the rack, in my case just 4 small bolts that slip into existing holes on the rack to hold the bars in place while loading the boats. There is a couple of heavy-duty hooks between the boats.

The top edges of the crossbars are customized to fit the boats: for the kayaks have cross grooves that fit the cockpit coamings so the kayaks rest snugly and cannot twist in the slipstream.

Once the boats are in place I run a couple of belts over them, between the fore-and-aft bars of the rack, passing under the hooks between the boats. The belts have rachet tighteners similar to the ones truckers use and these are available in hardware stores, quite cheap.

That arrangement holds boats on the roof at legal highway speeds. It has never shifted, but I run a line from the front of each boat to the front of the vehicle: in my case I added a pair of eyelets that pass through the plastic grill and are bolted to a structural member behind the grill. These lines are to keep the boat attached to the vehicle if the straps fail. At the rear end of each boat I attach a small flag that is visible through the rear mirror. I can now check the boats haven't moved at either end without stopping.