>Can anyone tell me where to find (or reference) plans to build my own rack
>for cartopping a single or a double?
Something that works well for singles (you could probably use a slightly larger
version for a 2X) is a piece of aluminum rectangular
tube 2 inch by 2 inch or 2 inch by 4
inch. The tube is a bit longer than the***pit of your boat.
Bolted to this tube at each end is a "V" with a flat bottom and a webbing
pop-riveted or fastened by some other means across the top of the "V".
The top end of the "V" at each end is (say) 6 cm wider
than your boat at that point.
The "V" is high enough to ensure that your boat won't touch the support tube.
Drill holes in the underside of the tube, and loop shock cords over your
hull at the V, and hook them into the holes. (The holes should be close to
the "V" at each end of the rack.
Of course, before you put the boat on the rack, use some form of clamping
system to hold the rack to your roof rack.
Hope this helps.