That's a brilliant idea. I think the electronics part is doable, a radio transmitter is a very simple thing. Some metal alloy with sufficient robustness would probably do the job.

We could even build an analog device that could store the messages just like a music box cylinder.

https://www.google.com/search?q=simple+radio+transmitter&tbm...

https://www.google.com/search?q=music+box+cylinder&tbm=isch

Also check out Graveyard orbits for decommissioned satellites where they can stay for millions of years and be our monuments from the 21st century.

https://en.wikipedia.org/wiki/Graveyard_orbit