Here's a link to mine:
https://www.etsy.com/listing/87269752/custom-leather-kindle-touch-case
I use 3 leather corner straps and an elastic strap.
Basicly, you measure the device, and modify it by ((2x(Width + 1/4"))+7/8") X (Height+1/2")
That'll give you a good seam allowance.
Good luck!