Hi all and thanks for your replies :-)
Finally I found an easy and very satisfying (at least for me) solution.
Amazon Ember is close to the "Open Sans" font freely available from Google Fonts :
https://fonts.google.com/specimen/Open+Sans
I downloaded the zip from google fonts, copied
- OpenSans-Bold.ttf
- OpenSans-BoldItalic.ttf
- OpenSans-Regular.ttf
- OpenSans-Italic.ttf
from the google fonts zip to kobo /fonts folder,
and it works very fine.
Reading with Open Sans is very pleasant (but, again, that's very subjective).
Here's a sample with "Open Sans" on my H2o (2nd ed) :
As you can see by comparison with the pic on my OP, it's looks quite similar to the display on the paperwhite with Amazon Ember, with no need for any tweaking (apart from copying the right files at the right place).