Cloud service support for Kobo v0.2
Installation

Copy the KoboRoot.tgz archive in the .kobo directory of your reader (it is a hidden directory, so you might want to enable the visualization of hidden files) and restart your Kobo.
Configuration

After restarting, plug your kobo back into the computer. Navigate to the .kobo directory. A new kobocloud directory should have appeared. Inside, edit the file kobocloudrc to add the links to the cloud services (one per line).
Notice: No subdirectories are supported at the moment, so be sure that your books are all in the same directories that you are sharing.
Nextcloud (Owncloud)

To add a nextcloud (owncloud) link, open your nextcloud website in a browser and select the folder that you want to share. Click on "Share" and select "Share with link". Copy the link into the kobocloudrc file.
Please note that you need a recent Nextcloud version. Owncloud is at the moment not tested, but should also work.
Dropbox

To add a Dropbox link, open your dropbox in a browser. Select the folder that you want to share and click "Share" and "Send link". Copy the link that appears into the kobocloudrc file.
Usage

The new files will be downloaded when the kobo connects to the Internet for a sync.
Troubleshooting

KoboCloud keeps a log of each session in the .kobo/kobocloud/get.log file. If something goes wrong, useful information can be found there. Please send a copy of this file with every bug report.
Known issues

No subdirectories are supported
Support of Google Drive can be considered if there is demand

Acknowledgment

Thanks to the defunct SendToKobo service for the inspiration of the project and for the basis of the scripts.
Curl for Kobo was downloaded from here: https://www.mobileread.com/forums/showthread.php?p=3734553 . Thanks to NiLuJe for providing it!
Thanks to Christoph Burschka for the help in updating this tool to the recent versions of kobo and nextcloud.