I thought about a 'Z" frame, to make the plywood rigid. Then drilling holes through the 3/8 in plywood, through the 2x4s, and then into the 4x4 posts, and still the wood would sag in places. So, I added a post, dead center of the Z, and still it would sag, and be troublesome. I tend to over think stuff.
Called a friend who is *infinitely* better at "keeping it simple.
"ok honey, you are making this too hard. Start with Rigid 3/4 inch ply. Cut to the length of your shortest section of your trunk.
5'7, for mine.
Rip cut it down the center, so the two stacked pieces fit in your trunk.
Use 9 posts, each as high as your highest Tote, that you are putting underneath.
Sure, three, against the wall, under your first piece of ply. 4 screws each.
Three middle posts, each with plywood, *halfway* covering your 4x4 posts, (two screws)
Next sheet of ply, Covers other half of tops of center 4x4 posts, and final three, on outside edge.
3" drywall screws, with starter holes, and thing should be rigid as all heck.
My post cuts left me with an extra 4x4 post, which I am going to hang the trimmed 5 inches of ply off, of, to give us "foot support" at the end of the bed.
If you are *tall*, and you need that 6' bed, you can always place a final section at the bottom of the bed, with an extra two posts, or not trim at all, if you have a pickup truck, etc.
You can reuse it every year. 63 dollars in lumber, including cuts, at Home Despot, and 5 bucks worth of drywall screws. I assume you have a drill?
I added (my own touch), 12 feet of 3/4 inch, foam hot water pipe insulation. It is split on one side, to wrap around the edges of the plywood bed base, (splinters in bed! Ouch!) Staple Gun it in place. 98 cents per section of foam insulation.