I had the BoatXFX ones, but lost a cover recently - posted this in the other cover strap thread. They stretch quite a bit, and I guess allowed a corner to lift and after that she went flying. I'm back to using velcro straps, same ones fisheye posted - the 1x30 works great on 12" units.
I've never been a fan of any type of bungee cords. I'm a retired ophthalmologist and have seen too many serious eye injuries from them. Just my opinion, but if your'e going to use them, I would suggest wearing a pair of polycarbonate glasses.