Walnut and Poplar

This morning I got out to my hard wood supplier to buy the materials I need for my tool case, which I'm almost finished drawing. I bought 4/4 low-quality walnut that was cut from trees by our local lake, and 4/4 popular. Neither boards are very wide, but it doesn't matter. As I was coming home, I realized I forgot to buy wood for the panel in the frame and panel door.
(I'm sorry about the picture being sideways, I don't quite know how to flip it.)

Here is the design almost finished. The large space in the top right hand corner is where my #4 plane will go, I just didn't decide to draw it. Same with the space to the left of my square, it will hold the bevel-up plane(when I'm finished). All the case will be is a shallow box, dovetailed at the corners, with a frame and panel door, and different tool specific holders. The rails, stiles, and holders will be walnut and the rest will be poplar. I'm not quite sure what to do for the back, I'm thinking about using plywood, since the holders will be attached to it, and if the back wood moves, it could mess up the holders.