You'll have to stand them off the deck then. It really is tight back there, and the trunk springs are also in the way. I'd be thinking hand-laid GRP would be the ticket, over some fiberboard. Or just cut an arc of fiberboard, like an ellipsoid or eye shape that pushes the rear lip of the speaker high enough that the magnet clears the decking.
Make it all match with some new fabric, it'd look great & sound even better.