How do you feel about some simple designs in each zigzag? I did a quick markup on my phone for you, so please excuse the unevenness!
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S curves, big wavy line, wishbone, ribbon candy, circles on a chain, stars on a chain...
Basically anything on a chain would look good and fill the space pretty quickly - a small flower, different sized stars, different sized circles (alternate small vs big?), swirl circles, or whatever you are comfortable doing. Kind of like a chandelier, in idea.
Some designs will be easier than others to turn the corner. I usually have to mark some "hit points" for myself to space out the design and hit the right spots with the right part of the design, if that makes sense. Sometimes it helps to just draw the diagonal line from corner to corner so you know where the center of the turn is.
Alternatively, you could just fill the space with straight line zigzags? Lots of options
I love Lori Kennedy's blog for ideas and she's great at showing how to stitch out the motifs. If you have time, you might consider some of these?
https://lorikennedyquilts.com/category/motifs/all/