Well, if you
really want a wrist strap... Take a look at the pictures in
https://wiki.mobileread.com/wiki/File:OpenKobo.jpg If you are really careful, you can drill a hole through the bottom left corner - no more than a sixteenth inch, I'm sure - between the battery and the edge of the case. Then you can push through a loop of thread fastened to your strap.
I'm not recommending that, mind you. I'm sure you wouldn't have a warranty left, for example, and it will allow dust and moisture to enter the Kobo.
Do I have to explicitly disclaim all responsibility?
Probably.