This Oaxis inkcase looks good, but it's a little pricey at $97, and also is discontinued.
Oaxis currently sells e-ink cases for iphones, but do they pair with android and display correctly? I'm aware they won't fit snugly on the back, but I'm less concerned about that.
Use case: bicycle navigation in a bike phone holder. Having the screen on burns through the battery, and also is hard to see in direct sunlight, even on max brightness. I'm using an S5.