It seems the game require a single ratio so what you can do is letterbox the game to force this ratio (say, 16/9). I've found no good resolution trying many, it's always cropped strangely.
I'm not sure I understood the question. I don't letterbox Masteroid, it runs at whatever resolution you specify. However, the menu doesn't scale below 1050 well. I will try to either letterbox lower than 1050 resolution or rework the menu. I was actually working on new features for v0.10 right now but I'm switching gears to fix this!