I have a front locker lid held down with a length of bungee. I drilled 2 holes in the lid and passed a length of cord through and tied knots. The loop then stretches over a plastic mushroom I made. You could use a few small then a large plastic or steel washer instead of the mushroom.
Second option is a tab on the cushion with a press stud that goes onto the mating half screwed to the locker. I have both as I made the hatch long before the cushion arrived and I don't always take the cushion with me.
Hadn't realised how filthy the locker was. Time to get scrubbing I guess.