Thanks for the responses.

Unfortunately, I can't do anything about the lack of a door--I rent this place and I was lucky enough to get the shed. I also can't coat it with WD-40 or anything like that since this is my commuter. It's an old 80s Bridgestone MTB that I got off ebay for $200. So it's a steel frame. But it's corrosion free as of now, and I'd like to keep it that way.

Is the bike guaranteed to rust if I leave it out there? The rust-proof cover somone linked looks interesting...