This is my third winter with one of these. Here's what I wrote after the first winter.
My winters are warmer than yours and our frost depth is only 30". I'm not terribly careful about keeping the top of the hole insulated, though. I'm still using a couple paper leaf bags to cover. Last year they had leaves, not straw stuffed in them. The bags just barely cover the hole, not much, if any, overlap of the surrounding ground. In your climate I could see laying some styrofoam down over the hole in addition to whatever else you use to help insulate. If you make sure you've got good overlap around the hole's edges, I bet it would work.
It turns out, the way I do it anyway, no ventilation is needed. I have a pretty high straw to potato ratio, though. The last couple years I've made sure each layer of potatoes has some straw separating it from the next layer. The potatoes in each layer don't quite touch each other. This year I had more potatoes and didn't want to change my setup to accommodate a second garbage can. So I've got more potatoes in each layer, all touching and piled a couple potatoes deep.
I'm just finishing off the damaged or otherwise unsuitable for storage potatoes, so I haven't even checked the garbage can yet this winter. I'll let you know how they look once I get in there.
I'm super happy with my set up, though. I ate my very last Yukon golds in July last year, after digging them at the end of September. They were sprouting a bit, but still good.