Have a p229 sig with no upper slide. Probably be a few months before I pick up a slide. How should I properly store it for 6 months or so.

Clean and lube as you normally would, wrap in an old t shirt, and put it away in a drawer or safe. Unless there is going to be some unusual exposure to humidity there are no special prep needed.

Use a lube that is known more for its protective qualities like Easox.