I use Stack from TransIP for my Duplicati backups. I’m not sure if invite requests are accepted globally, it’s a Dutch storage service.
TransIP the biggest hosting company in the Netherlands. With Stack you get 1000 GB (!) storage for free, based on OwnCloud. It’s completely free, no ads, no restrictions and can be accessed using WebDAV. So it’s a great place to use as a backend for Duplicati.
You can request an invite here: TransIP | STACK - Van jou: 1000GB gratis opslag