rockm

"Would it be a waste of time to make a small "greenhouse" type structure, or should they be ok as long as they are sheltered from the extreme cold and wind?"

Not only would it be a waste of time, but it could complicate your storage. Enclosed Greenhouse structures, or those that allow light into a closed environment, trap heat. This is a bad thing for overwintering trees, as dormancy depends on the number of "chilling hours" they require to complete the dormancy process. In other words, they need cold, but just nost drastic cold. Structures aimed at trapping heat can cause premature bud break, and premature growth--you don't want your maple pushing new growth in early February...

That said, I use a clear plastic "greenhouse" covering over my cold pit, but the structure's sides remain open for the vast majority of the winter. It is placed in complete shade all through the winter to prevent heat build up. I close it only to keep out torrential rain and too much snow. It is not to retain heat, as much as it's for keeping out too much moisture.