Did it. It's still drying, but it looks like it's going to work perfectly. And no velcro required.
The lid closes down on top of the pre-placed waterproof plate.
Oh, and yes, the mrs decided it needed more pink, so the top is orange (when it's closed), and the bottom is pink - when it's open to put stuff in, and for when the dogs are in the back.
So that's about it. Might a couple of handles on the lid to make it easier to open from the top, and to give it more tie-down positions. Maybe some more orange - but I'm used the the orange-camo pattern now so I might leave it till it's a bit scratched up first.
Might do the side steps orange to match when I put the checker plate on them.