Follow up ... my response not quite on target ... didn't mention auto backups, but same idea ...
mount point: /autobackups - points to the Amazon Bucket.
Moodle will still use moodledata/temp/backup/ to build the course backups. Last thing the code does is 'copy' the .mbz file in moodledata/temp/backup/[somehashnameddirectory]/ to the destination ... in this example that would be /autobackups/ ... the mount point for the Amazon Bucket. Moodle then cleans up moodledata/temp/backup/X ... so there is a burst of space usage there for a little while. Obviously if you are really in a bind for space (drive 99% full) then you'll have to find space.
I do use my Google Bucket in that fashion ... also use it for site backups. Nice thing about that ... if one of my servers crashes and I have to spin up another, I can set up the GB mount on new server and have both the auto course backups and the site backups to restore.