Given the fragility of the 3rd generation "Kindle" screens, a stiff case can make all the difference. This design can serve as a template for your own custom case (see my showroom photos for some examples of text and illustrations for engraving), and can also be used as a notebook case, etc. The design features some wiggle room, I use cork to hold the device firmly in place without rubbing the screen on the case.
I designed for a snug fit, erring cautiously to avoid the material tolerance causing the slots to be too small for construction, so a few minutes of sanding will speed along the construction process.
I cut this in 2.7mm bamboo, but you're welcome to try other materials. . .