Finally I managed to fix the error when adding the plugin. The error was in the first line:
# -*- coding: utf-8 -*-
For some reason, this line appears in the plugins packed with Calibre, but it's not recognized when adding with "calibre-customize". Well, saving the file in utf-8 and adding the plugin works!.
I attach my custom Boox reader plugin in this message. I hope it's useful for other users.
Thanks for your patience and help, kovid and user_none.