The ony thing different between what you want to do and what I did i the poor man system is I use a live backup (and a storage site) you just want a storage site. In that case, any storage location is fine. You problem only arises in the ability to automatically sort the backup files to keep the only the most recent. S3, like Dropbox, is only a storage location and not a full linux environment, This means that you cannot run the sorting scripts and the cron jobs from within S3.
It is only a storage location that you would have to go into to sort the files yourself. They may have some rudimentary ability to delete all but the last X number of files.
The scripts on the host would remain basically the same. The only thing you would do is replace the “scp” command with whatever command you use to transfer the file to your S3 storage.
Again you woud have to work out how to keep them sorted on the S3 box.
Altenatively, you could also get yourself a very cheap OpenVZ server that only has 1gb of memory and maybe 30gb of disk It would be able to run the sorting scripts and keep very good track of your backup files (even if there is no erpnext system there to use them. It would still be just happy storing and sorting once per hour. I have a similar server that does nothing but receive the files and sort them in case I need one someday. I tink it cost me about $24 per year for that particular server. I have only ever had to recall a file from it one time in the past year, but that one file saved me 6 days of rework so that one event was worth the $24 I paid for the year.
I use this storage server to hold the backups from 4 other erpnext sites. I give each one a directory in the default user home directory and the sorting scripts take care of the rest
As always, Your mileage may vary…