Step 5: Adding storage benches
The frame of the benches is constructed of 2" x 3" beams, and then covered on the sides with flooring material and attached to the deck by driving screws diagonally through the bottom square of the bench directly into the deck. The cover then has it's own frame made out of 2" x 2" beams with more flooring screwed on to it in top.
To attach the cover and install hinges, I added a 2" x 3" beam lying on its side and running along the entire back of the bench. This hinge beam has to be very strongly attached as you'll have the seats swinging off of it (and they're not exactly light). I glued it down, added metal brackets and screws on the front (one every foot or so) and 3" screws running diagonally through the back and into the sides of the bench (also one every foot or so).
Once the hinge beam is set up, measure the size for the seat frame, and then just make a square made out of 2" x 2" beams. Attach flooring material to the top of the frame and you're done. The hinges I used were regular door hinges that were safe to use on treated wood, out of the same concern with the ACQ approved screws. I just chose steel hinges that were coated in weather resistant paint.
Attach the hinges to the seat covers and hinge beam (recess them to make it a bit nicer and have the seat cover lie right next to the hinge beam), and you're done.