When I was a kid, and still rode shortboards

I would set the board flat on the seat, with the fins "boxing" the front of the seat. So you're sitting on the tail. Not sure how that'll play out with an 8'+ board. Putting the board on the side of the bike seems to be the thing in places like Bali, but any place you have to travel on main roads and at speed with other traffic, I think it's asking for a lot of trouble. A gust of wind could spell the end of you