The end panels are 1x3 frames Domino'd together with the pegboard in a ¼" centered dado, much like a Shaker-style cabinet door.
The bottom and top are ½" birch ply. The back is ¼" birch rabbeted in.
I made the units 18" deep for a couple of reasons. First, I was worried if I made the unit as deep as a Systainer, it would be too narrow to be stable. Second, I had a bunch of 18" drawer glides laying around, but they were only three-quarter extension glides so they only came out about 15-16 inches. The Systainers need about that much to open the lid without taking it off the tray, so happy coincidence.
The trays are just ½" birch ply with a 1x2 front edge, and a notch cut out of the back. Using 18" glides leaves a 4" hole behind the Systainer, so I notched each tray in a big U-shape to leave space behind the drawers. There is a ½" x ¾" lip that keeps the Systainers aligned to the front of the drawer. The tops have 3 slots near the back edge so I can slide in 4' levels, guide rails, and thin stuff like that. All of the trays are notched so the guide rails can slide all the way to the bottom.
I spaced the drawers a little further than necessary so that I can close the drawers without locking the TLOC. I'm lazy that way.
If I were to build it again, I'd probably use ¾" ply for the top and bottom. I haven't had any sagging issues, and I used the ½" because I had a bunch of offcuts from another job, so they were basically free.