Get one of these and you can get a third attachment point from the front of the car and to the leash string. I would use something with some elasticity too. Should keep it more secured. You could also get a board bag and fashion attachment points into the bag with a grommeting tool. Otherwise buy a real roof rack and dont worry about much
I took my canoe on a decent road trip. Had no issues, car handled it well. I didn't have a back tie down, but used straps similar to these Thule one to strap down to the hood. Good luck!
I've been using this for ~9 years now. Very solid.
Thule 530 Quick Loop Strap, Black https://www.amazon.ca/dp/B001EOZD26/ref=cm_sw_r_awdo_navT_g_D6KF7Q1Y5VNBQM1QJA92
Thule Quick Loop Strap https://www.amazon.com/dp/B001EOZD26/ref=cm_sw_r_cp_apa_i_iVikFbBTJZ1AS
Get some of these
Thule 530 Quick Loop Strap https://www.amazon.com/dp/B001EOZD26/ref=cm_sw_r_cp_apa_LKZPxb0WFWS33