New window in play house

Hi, I'm building a playhouse for my kids - the top having a closed in fort area. I want to add a couple windows, and found shed windows 12x18 for new construction with a nailing flange. Framing seems simple enough - I just have one question really. Regarding the rough opening, and then using shims... since there is no interior wall - so no trim or anything can I make the rough opening the exact size of the window - and then just slide it in, screw the flange in and be done? Any advice appreciated.