this looks really interesting ..i was looking myself for an inexpensive system for a toyota prius.

i would go for it but definitely i would rig up a proper bow and stern line .  for something like this my main concern would be to ensure my bow lines are in a triangle to ensure that the bow of the boat stays pointing forward and to ensure down pressure.  same for the stern line.

i use a system like this https://www.ebay.com/itm/Quick-Hood-Loops-Trunk-Anchor-Kayak-Tie-Downs-Straps-Bow-Stern-Canoe-Transport-S/392294813573?_trkparms=aid%3D555018%26algo%3DPL.SIM%26ao%3D1%26asc%3D59444%26meid%3D93d980f9c47f4300a4edca0ad2d45954%26pid%3D100005%26rk%3D1%26rkt%3D12%26mehot%3Dco%26sd%3D392257708413%26itm%3D392294813573%26pmt%3D1%26noa%3D0%26pg%3D2047675&_trksid=p2047675.c100005.m1851

on my car for the bow.  because the lines come from either side of the hood....they ensure the bow stays pointed to the wind. 

h    

You should use a Yakama or Thule designed rack made to securely hold the boat on the roof of your car.

. But then if you want cheap there is this,

   https://www.westmarine.com/buy/west-marine--deluxe-universal-kayak-carrier-kit--13850326?recordNum=11

I bought a set of hood loops, tie down straps and roof top carrier pads, all from CLC, when I built my Wood Duck at one of the CLC classes. They work fine for a single boat. In the past I have used the J style kayak carriers attached to my roof racks but found it difficult to attach a boat securely. Carrying the boat on its side also makes it very high when trying to arrange straps. If you have two boats it was almost impossible to manage the straps and get everything tied securely. My car is a 2017 RAV4 hybrid, so it’s already on the tall side. Smaller, plastic yaks are probably easier to carry, but the bigger plastic fishing yaks are heavy and hard to tie down. The pads and hood loops are lighter, easily removable and less expensive.