I'm not sure if you have Costco in the NE, and even if you do you may have missed the annual supply of portable garages, but theirs is fairly high quality, cheap ($250-ish?), 10x20, and does indeed open in the middle. You could load from either end and the middle.
Would you blow snow off regularly to keep it from collapsing under the weight? For a less labor intensive long-term solution, some of the admittedly more initially expensive ideas in this thread may be useful. But Costco has what you want if you still want it.