Actually, there is a x2 button to do exactly that. It's the one in the far right lower corner. Probably, the icon isn't recognisable enough. We'll put it on our to-do ^^
I did the same thing, just designed for 16:9. Knowing people would have mostly would play on 1920x1080 screens, I locked the build to only play in Windowed mode at 1600x900. That way the UI should work fine for anyone who played.
