"You can buy a used shipping container for about $1100"
Nope, expect twice that for a 20 footer delivered. It was for me.
I greatly value the shipping container as a deterent for vandalism, theft, or unauthorized use. I believe the liability of leaving the tractor outside, even locked, is substantial.
You can not stop a determined thief, the best thing is to prevent the casual oppurtunity thief from easy pickings. If you must leave it out there in the open, try to find a good hiding place and maybe some of that camoflauge netting to hide it well.