No, 'window of opportunity' doesn't sound too well when translated literally («окно́ возмо́жности»). While some people use the loan translation, it still sounds somewhat foreign and it's better to re-phrase it without using «окно́».
- Э́то о́чень ма́ленькое окно́ = It is a very small window (or "this very small window", but this is not a complete sentence).
- Э́то окно́ о́чень ма́ленькое = This window is very small.
I just read an article about small windows/holes used in Italy to serve wine, both in times of bubonic plague and coronavirus. They are also very small windows. https://edition.cnn.com/style/article/little-wine-holes-buchette-florence-coronavirus/index.html