Here are some photos showing how I am using the cedar fence planks. I have a post anchor that goes 2’ into the ground (much easier and less permanent that pouring a concrete footing) which holds the pressure treated 4x4. I then bolt a short fence plank length on the sides of the 4x4, and cut a nice angle on them. The 5.5" wide planks are then screwed from above into these lateral pieces, and I put a few more laterals on the bottom of the planks to keep them together better. They cup a little bit, but across the depth of the bench, they are fine. One of my benches is 2 boards deep (11" total), and my longer one is 3 boards deep (16.5").
You can also wait 3-6 months until they do their initial cupping, and sand the top down back to flat. Just make sure your screws are recessed enough. They shouldn’t change much more than this initial movement. I haven’t had to do this yet.
For this one, I used all redwood.