This is the cheapest and easiest way to do it. Still plenty sturdy.
For the rear strap, I got some D-rings and screwed them into the rear most coat hooks. Then looped a sturdy tie down from d-ring to d-ring.
For the front strap, I just looped a tie down from the rear seats grab handle to grab handle.
My boards are shortboards.......the longest being 5'9". Not sure if a longboard would fit there and still give you enough head room, but it's worth a try.