Not sure of the area around you, but post an ad on Craigslist. You might be able to find a farmer, heavy equipment operators etc, with a barn or building. The favorite spot for the classic car guys in my area, are abandoned Nike missle silos. You also might check with a Comercial real estate agent in your area, large spaces may be empty awaiting sale or lease and some rent out spots.
Security is always a concern in any storage situation so look before you leap.
I tried the cover and it created more problems than it ever solved.