I want to create an artificial stratosphere for my crop, I might grow some cucumber. Earth has stratosphere because of gravity, but how do I create one in the thin air? Open to ideas, many thanks

What do you mean by artificial stratosphere? The stratosphere starts ~10 km above the surface of the earth (depending on your location) and has a particular temperature and pressure profile. What are you hoping to recreate?

Like earth, where it is surrounded by stratosphere naturally, maybe create a stratosphere like that around the spaceship, that way we won't ever have to worry about air escaping into the vacuum. I know so far there isn't a way to create artificial gravity like that, only the one with mechanical force, but maybe with other means it would help, I haven't thought it through, that's why I'm asking for ideas

You need to formulate the problem better than that. ("What spaceship?" is but one issue; there was nothing about a spaceship in the OP) It sounds like you mean atmosphere, but OTOH, it doesn't.