My original idea for the fill in rectangles was to use the same fabrics that I had used for the borders of the blocks. This shows the area partially filled in. (This post is a continuation on design from yesterday in case you didn't read that one.)
I thought I might like a band of color on each edge of those rectangles and only cut 4 strips to try it out. Red is not the right color; it probably needs to be a dark tan. I don't think I like this now that I see it.
I tried a darker beige print for the rectangles but I didn't care for that either, especially that the fabric was all the same instead of a variety of prints.
I thought maybe a stripe would work; not sure about this one yet.
Here are all of the choices the way my design wall looks right now.