I fit 2 boards and loads of stuff in the VY, locked in. But you can get sexy stainless steel fittings to attach neatly to the hard cover and then put s/steel bars across or just strap the boards down on top of the cover. Boat shops have the fittings, the ones they put in decks.
Or, when effecting movement of boards of extended longtitudinal format, I let them poke out the back and have a long occy strap holding the lid down, works ok, as aus301 says. You can get the occy strap cut to length from bunnings, and the ends which just tie in, easy, cheap, and when not using it, you cant see it.
Can also use the strap to tie your wife up, or flick your mates in the back of the head.