Unity3d How to support different screen size in Portrait orientation

So, I know in Landscape orientation game, I can create an background which is wide enough to accommodate the widest possible screen(like 16:9). The extra background for wide screens is simply ignored on the other aspect ratios.

My problem is, my game is in Portrait mode(like the Space Shooter game), I want the unity to cut the extra background in vertical direction, not the horizontal direction. That is, currently, the unity will ignore backgrounds in left and right on some screen size, I want it to ignore backgrounds in top and bottom on some screen size.

I don't want to resize camera aspect. So how to do it? thank you!