I bought a strap similar to this one
https://www.ebay.com.au/itm/32372404...Cclp%3A2047675.
I can't recall the exact size but I mounted it diagonaly and it make it very easy to hold, you don't even have the grip the reader, it holds the ereader with your hand wide open.
It also make it really easy to use the buttons as well.
Best investment for my ereader that I ever made.