Host media smartly on another host (DIY CDN?)

I’m looking for a smart way to do a ‘hobo’ CDN of sorts. I have some sites with lots of pictures, media etc but they are hosted on a managed host with limited disk space.

I have some other servers that I can easily store tons of media on free, so I was hoping for a way to automate transfer of media to another host and update links in WP to point there.

Ideally in a smart way so that new media is automagically uploaded to the imagehost and possibly rules for what to keep local and what to upload.

Does such a thing exist or are there any clever tricks I could try?

Thanks!