[quote]
I'm hoping the bungees through the foot strap and around the boards will keep them in place.
[quote]
Please never, never, rely on a bungee for safety (ever). Not on top of the van, behind it, or inside it. A short length of Sta-set or spectra cord (any good climbing or sailing rope) with a simple bowline knot through something securely fixed to the boards and tied to the frame or a folding pad eye in the back.
The seat may protect you; it may not. It was not designed to protect you from flying objects from the rear. Even though the boards are light, they will hit with some force if they are traveling 50 mph and you are standing still. Simultaneously, the airbag will be flying at you some extraordinary speed.
Also, your passenger may be taller than the top of the seat and they may take his/her head off or the boards may fly through the front window and injure the other guy.
Rope is simple, cheap, effective. Sorry to preach but I've been in a full speed head-on on an interstate highway with lumber in the back (not a smb). You don't want to have these things tied with a bungee.
Tom