What are the most flexible/elastic resins for printers? Are there any easily accessible additives that could make standard resins more flexible?

Title. I'm looking for resins with the lowest Shore hardness, the lowest one I've found (that I can buy on aliexpress) is Resione F39, with Shore 60-75A. Ideally, I'd want to get something closer to Shore 30-40A.

So, failing that, anyone with a higher understanding of chemistry can explain what additives are usually added, resulting in a cured resin with greater elasticity?