I notice that there has been a questionnaire added to the
https://updog.co site asking about using Cloud services other than Dropbox in conjunction with updog. The question of what cloud services can be used for a Calibre+Calibre2opds combination is one that frequently gets asked and up to now the choice has been very limited.
I would encourage any Calibre2opds users who are interested in using alternative cloud services to host a Calibre+Calibre2opds combination to put in their vote to encourage the updog developer to put in the work for getting their favourite cloud service supported via updog.
Disclaimer: I personally have no link to updog.co - I am merely promoting it as a way to make Calibre2opds useful to a wider audience.