Why not cover the greenhouse with a tarp? Then you will have a shed.
I kept some last winter in a friends greenhouse that is more like 30x50. I took them in before frost (the important ones) and then reduced water to stop growth and help them drop their leaves. After they were dormant I put them back out in Nov to go fully dormant, then back in before temps were below 25 at night. Opening the doors on *most* hot days to keep the temps down. In late Feb when it was warmer they went back outside and began getting repotted. I was happy with the results and only lost one, which was a young airlayer that died because I left the door open on accident overnight so all of the pots froze, oh well.
Another option that is not often discussed is to dig a 5ft deep pit and put them in there, cover with plywood and a tarp. I think it was Herman2 who used to do that but not sure...