Thanks for your time and effort trying out covers--and sharing the results. I'm gonna try a SOFT BACK (TPU) standard flip cover for the iPad 5, because it includes a slot for the stylus, but I'm weary about it because the 5 is a bit longer than the iPad 1-2-3s.
Case I'm going to try:
https://smile.amazon.com/gp/product/B07VR3NXGF/
Device Measurements for reference:
Boox Nova Pro
196.3 × 137 × 7.7 mm
7.73 x 5.4 x 0.3 in
iPad Mini 1
199.9 × 134.6 × 7.1 mm
7.87 × 5.3 × 0.28 in
iPad Mini 2 + 3
199.9 × 134.6 × 7.4 mm
7.87 × 5.3 × 0.29 in
iPad Mini 4 + 5
203.2 × 134.6 × 6.1 mm
8 × 5.3 × 0.24 in