Storm window??

Unless you can build something that seals out the weather all the way around the window, covering the outside may not help much.

You want to create a sealed, dead air space between the cold area and the heated area.

You might want to consider either a custom storm window insert from some place like this:

or something like one of the products found here:

Is the "garden" in the window still active in the winter?

I'm not sure I understood DD's comment.......... but something to seal off the space from the inside would be more effective than a cover.

Or you could rebuild (or have rebuilt) the window system using double pane panels.

cheers Bob

Cheapest (and ugliest) solution is to simply duct tape a double layer of plastic film on the outside and one on the inside if you can, while you make plans to have some structural improvements made next summer. Obviously, a better design will pay for itself in a couple of years or so. Good luck.

Joe