I ended up getting an electrician installing an outdoor power point.

Costed $160 all up. Happy days!

Thanks for all the inputs.

danepak:

Thanks. Would anyone have a rough rough idea of how much an outdoor power point would cost?

Also, our switchboard is located in the middle of the house, unfortunately not outside.

As we're building, we're not 100% sure either, where a good location for the outdoor power plug would be.

 

In regards to my original plan.

I was thinking plywood (or similar) cut like this with a small hole.

As we're in Auckland, it doesn't get mega cold in winter. Therefore, a combination of this (see image, where grey image could be plywood) to cover most of the window + curtains, would keep us pretty warm at nighttime.

Note that it's half a hole, so I don't need to pull the entire plug through, but just cable.