It seems like it might make more sense to make a vent on the bottom because that will allow you to add in heating devices if that is necessary, and I don't think CO2 deficiency will be a problem if your banana is going to be mostly dormant through the winter.
Here's a blog on my solar greenhouse which has a solar vent, and a zip open door. It is insulated a lot more now along with a large compost pile in the middle, but this setup alone can raise it from a -5F night up to around 15F. It doesn't take much to keep it above freezing in there. I usually keep the door open when it exceeds 80F which can happen even when the outdoor temperature is 50F.
Steve's garden: Setting up the Solar greenhouse