Build an extender frame to extend the length of the roofrack bars beyond the width of Your boat. This is basically "four planks nailed to a square". The longitudal planks set the distance between the roofrack bars. Set this to the maximum Your car rooftop allows. The crosswise planks will rest on top of the roofrack bars, and the boat in turn will rest on top of the crosswise planks. When first assembling the frame allow some extra length on the crosswise planks, say Your boat width plus 50 cm (2 ft). Cut the extra off only after fitting the boat to the rack.
The frame will be held in place by the U-shaped pieces of plank on the underside of the frame. These pieces need to fit exactly over the ends of the roofrack bars.
The boat again will be held by pieces of two by two on the topside of the frame. These pieces need to grab the edges of the boat firmly so do not attach them to the crosswise planks blindly. First put the frame without these pieces on top of Your roofrack (on top of Your car), then put on the boat, and only then screw on these pieces.
For the rear suport frame You need a bike rack that clamps onto a trailer ball. (You need to have a trailer ball on Your car.) Remove the bike support horns from the bike rack bars. Get two lengths of tubing that exactly fit on (or in, for a light weight version) the original bike rack tubing, to extend the height of the bike rack to reach the stern of Your boat on top of Your car. Bolt the tubing to the bike rack, through both tubings.
Drill holes through the extension tubings 2-3 cm (an inch or so) apart. I drilled 7 mm holes to later accept M6 bolts (around 1/4""). Paint the extensions.
You can, of course, first fit the boat, mark the correct
height for the crosswise plank that comes on next, then
remove the boat, remove the bike rack, drill only those
holes You need. But this involves one extra boat topping
untopping operation.
Or drill the holes with the boat on top, after painting
the extensions.
Put all the frames on the car and the boat on top. Fit a horizontal plank to the rear support frame. This plank should extend about 10 cm (4") beyond the gunwale on either side. Note strap slots on the lower side of the plank.
Bolt the plank to the tubings with bolts and wing nuts. (You will need to remove this plank every time You remove the bike rack, that's why wing nuts.)
A length of two by four about twice the car height is a big help when topping and untopping the boat. It acts as a slide, protects the boat from accidental "rolls" and protects the car from the boat. The two by four should be carried along with the oars on top of the roofrack underneath the boat. You will need it "at the other end".
A 6.5 m (21 ft) boat rooftopped on a 4.6 m (15 ft) car. Looks scary but is actually very stable and feels safe to drive. This combination has happily survived 1600 km (1000 miles) mostly at 100 km/h (60 mph), with occasional 120 km/h (75 mph) on the motorway.