Camera size and GUI elements

So i am making a game where i basically want the camera width to be constrained to its height (ie. a 1:1 aspect ratio). However since this wastes a lot of screenspace, i want to devote the remaining screen space to GUI elements.

I was able to get this result:

however, i want the side panel to be sized according to the remaining screen space so i think i'll have to do it through code.

 

Any thoughts?