I liked Apple’s Smart Cover to protect the front personally, and always used a gelskin on the back. The skin was a bit for looks, but mostly I greatly preferred the extra gripiness it provided, along with not being cool to the touch in winter.
https://www.gelaskins.com/ Not exactly what you are after, but thought I’d mention it.
Also, I’ve often wondered this about wooden table covers too. But if you are putting a 1 piece back cover on to protect the back...what are you protecting it against? Not solid drops/hits as you’d likely be worried about breakage of the screen too in this instance. Scratches? You want to keep the back looking really cool with no scratches....yet you never see that really cool back because it is always covered...
Not criticizing you, just trying to understand your use case. Like I said, I’ve seen people buy beautiful wood tables to only end up leaving covers on them 99% of the time. One woman I know spent $40k on a wood floor with intricate inlays only to cover up all but a foot around the perimeter with a rug. I’m sure I’m missing something, lol.