We built these racks for a john boat but they worked great for canoes, kayaks, and SUPs.
It is just a 2X4 cut to the right length for the roof and then attached to a suction cup handle with very heavy duty zip ties.
It rack has no trouble with heavier kayaks or other light boats.
This design only keeps the load off of the roof. To keep the load and the racks on the roof you can see that we put straps though the doors and tightened then very snug. ( Open the doors to strap down the load).
The suction cups are not what is holding this load down so please don't do something dumb and blame me. In fact I'll just say don't try this at home! I just think it might be neat for others to see what worked for me.