Like Pete said, a third strap over the two boards will keep it secure at the front. The strap will be fine around the boot hinges on a three door, just make sure that you close the boot almost fully when you do the final tightening. If you're a bit worried, something around the two boards will keep them stuck, especially through the board bag handles.
Those straps are great, easy to use on trips abroad with rentals and Moroccan taxis - I've also had a sofa down from Bristol on the top of a Fiesta and a double mattress on the roof of my Clio using them... versatile
Though don't forget the twist to keep them quiet.