I think I would put the kale evenly spread apart in the back, then put the lettuce in between and around. Kale could also handle the reflected heat from the box behind it better than the wilty lettuce. The red lettuce looks good next to the frosty blue green of kale. Remember, too that the lettuce is going to be harvested and gone, then you will be using the space for something else -- maybe bush beans.
Strawberries should be planted 12 inches apart. You can plant somewhat closer if you control the number of runners you allow to grow and touch down, or if you don't intend to let any grow at all, but each strawberry plant will easily grow 8-12" long roots and they tend to be shallow. In fact, if that back box is all the way deep and if there is not bottom to these boxes, then I would use the deep back box for something other than strawberries. Strawberries can go in the front box.
The shallower rooted lettuce could go in the upper boxes, even the top box between and behind the shade of bigger, deeper rooted plants like tomatoes or eggplants (or cucumbers for example) and be harvested and gone by the time those plants grow enough to want the space. You could also grow things that need the depth like carrots in the top box. In the scheme of things, peppers are somewhat shallower rooted so they could probably go in the middle box.
(I know you have another thread about your garden. Was there a reason you made this thread separate from your other thread? It might have been better to discuss all this in the same thread. I can't remember what all you are planning to grow.)
I would put medium tall/root depth plants like basil in the middle box. I would actually put the kale in the middle box.
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