The Hercules wood slats are intended to be held down by double-sided tape on top of the gray cross-pieces. The sizes of the slats do not allow them to fit in nicely, at least one needs to be cut into a narrow sliver to make the last gap. And that tape surely would've failed eventually. The overall width is 7.9" - and I found no standard type of trim, molding, or craft sticks that would work out correctly.
Eventually I found this lath material, and it wasn't perfect either, but it was cheap and kinda rustic-looking. It doesn't have a scale appearance, but so be it. With some edge-sanding of 2 pieces I was able to fit two 1/2"-wide x1/4" aluminum runners in the spacing.
So I made the wooden cross-pieces for gluing/screwing the lath onto. Like most things, it wasn't so simple. I had to review the spacing on the cross-pieces vs the gray ones, vs screws coming up thru the bed plate, and vs the screws for the cargo rail I added. But fortunately, I quickly got a reasonable result and put the slats down, piece-by-piece, over several days of glue-drying. And each one needed a wiring slot, of course.
The lath was bending upward at the rear, so there's an extra row of screws there.
The lath was also a benefit as narrower wood would've required even smaller screws, and would've been tough to drive in a structural use like this. (these screws are #2)
https://lh3.googleusercontent.com/pw...-gm?authuser=0