Keep screen width instead of height for resolutions

I don't post this as an answer, because I havent tested it very well, but I think this might push you in the right direction.

You'll want to change the camera's orthographicSize). You want it to have a value like 5 (can be anything you want) for instance on your target aspect ratio (height/width of screen)

As an easy example, you have everything set up for a resolution of 300x300 (aspect ratio of 1 as this makes further calculations simpler) on which you use 5 as orthographic size.

And you want to display it on 300x600 (aspect ratio of 0.5)

You should set your camera's orthographicSize to: 5 / aspect. So in case of 300x600 this would be (5 / 0.5 = 10)

Now if your original target aspect is not 1, you'll have to offset the aspect so that your target aspect is inserted as 1 in above calculations. But as I said, havent tested this enough to give you the exact maths ;)