What missing window? Acme window and door just finished repairs.
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Mark
Nice! Problem solved. Now if only it were that easy.
Well, you could consider the approach of using rubber molds to duplicate one of the existing windows as real-life Photoshopping 
If your kit didn't come with any extras, you're basically dependent on someone kitbashing another of these kits into a smaller structure needing fewer windows than the as-designed building (or enlarging it to a degree less than 2x the original).
If I were in this predicament, I'd opt for the duplicate with mold-and-resin tactic, unless the kit did normally come with extra windows (or the manufacturer keeps extras on hand)
---PCJ