I live in eastern Ohio,so I get the same weather you do, I just get it a day sooner.
I store my car in an unheated barn on a dirt floor. I bought a CAR BAG and stored my conv in it last winter. I took out the battery,put in 8 2.5 lb bags of dessicant, comes with the car bag,and zipped it up. When I pulled it out this spring, it looked exactly like I put it in,NO rust,NO dirt,NO rotent problems.
It's not too often that a product works as good as it is advertised, but this one does.
The one thing I don't like is that the bag is gray and you can't see through it, and you open it or you will let moisture.
What you are doing, will work but it is alot of work that could be avoided. The bag is 229 dollars, I think is well worth it.