Finn,
My own opinion is that you might be wasting your time and money.
At temperatures around the freezing mark, air holds very little moisture which is why your dehumidifier doesn't work that well. So don't worry about moisture.
If you "tent" your car, you will just be trapping ambient air and moisture in place. I don't think that's a good thing to do.
Silica Gel is a desiccant that is used in the canisters you talk about. In the states here, in bulk it is used in compressed air drying systems, and it costs about USD $50 for a kilo or two, which is a good amount. Problem is it works so well you'd probably spend all winter drying it out in your oven.
Get a good car cover, cover the car up in your garage, and don't worry about it. When you have some dry days, regardless of temperature, take the car out for a good drive, then put it back.
I put my car down for the winter in November, and take it out around April. It is not stored nearby, but when we have some dry days with no snow--and we do have some in the winter as I know you do as well--I'll take it out for a drive. This is maybe once or twice in that time period.
I think the worst thing you can do is to start it for a while and NOT drive it.
If you are really concerned, find some indoor, heated storage. It is very easy to find here, as long as you are willing to pay about $150-$200 per month. I imagine in other places it will cost a bit more.
Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored