[cajhin]

I'd like to offer a quick and 'good enough' fix (after spending entirely too much time pimping my Kobo fonts):

Simply drop the bold variants; copy only the Regular and Italic files.

The Kobo (at least the kepub viewer in the latest firmware) will generate bold and bold-italic from the base fonts. It looks ok, just like when you go to the Font Genius and increase the weight. Should be good enough for anyone who is not too picky about typography (most books contain little bold text anyway).

A hand-designed bold font is a bit nicer in the details, for example, an 'ffl' ligature like in 'waffle' may look a little off when bold-italic is generated from the base font, but otherwise it's not an obvious difference.

You can also drop the italic font, but the calculated italic looks a bit funny, especially the 'f' (it really needs to be longer than the regular f)

Thanks a lot @ jackie_w. I've tried many things with FontForge, but I never figured out that the OS/2-Panose-Weight attribute is the culprit.


Update: this only works for .kepub, not for .epub.
So, if you sideload ePubs without converting, you need to follow jackie's instructions to get proper bolded text.