I had the same problem with the shipping container I bought for storage (should have bought an insulated reefer, more $$), R-18 insulation, a plastic film vapor barrier some 2x4s and the cheapest paneling I could find, solved the problem. Just had to bite the bullet, spend the money and do a little work, and problem solved. You should do something, even if you go with a space heater, your bike will prematurely corrode in there without regular attention.