I wasn't sure whether to have the mattress rest on the actual walnut boards or tucked in, but I think I'll go with the latter (which is, the left sketch):
I started by planning all the boards to 1 1/4 inch thick,
Then I jointed all the edges. I don't have a jointer large enough to do this, so I just did it on the table saw. It's good enough for what I need it to be,
8 feet is as long as I can rip in my small shop...
Finally, I sanded everything to 220 grit for now. I'll do up to 300 later,