A sarcophagus filled with insect jelly and lined with intricate arcane conduits allows for indefinite suspension of living beings. Medieval cryptosleep replacement.
The Entosomnia Casket, closed and open.
Because the vanilla cryptosleep casket requires you to have a hi-tech research bench and a multi-analzyer, both of which you obviously can't get in a medieval playthrough, I opted to make the research cost much higher at 5000. The materials used are roughly on par with those used in vanilla to build a cryptosleep casket, at least in price.
These caskets are functionally identical to cryptosleep caskets, just with more flavor and a medieval research project. Colonists will still get cryptosleep sickness and cannot eject themselves.