I ended up using #6 1-1/8" black wood screws. They fit the cutouts in the speakers perfectly and the black makes it all tidy
One thing I am still hung up on is a more secure mounting method. I am hesitant to drill through the magic box and into the vinyl hatch corner. I'm thinking of fabricating 2 brackets that would mount to the edge of the corner trunk molding (see pictures). My goal is to have something that will hold the speaker securely so it won't fly around. Drilling into this trim/edge piece would not be visible when the carpet is covering the mounting point. It would also be a lot easier to remove the sub if needed to access the spare/tools.
Here's where I think would be the spots to fit the brackets to:
