I went the other way with my home built screen. My available vertical dimension on the wall is fixed and i wanted the biggest picture possible, so I made a 2.35:1 screen.
For 16:9 content, we use the full height of the screen, but the 2 sides of the screen have no image projected on them.
When we play 2.35:1 images, I manually zoom out to the full width of the screen, and the black (dark grey) bands top and bottom project off the screen onto the wall above and below.
I had originally planned to make black felt curtains for the sides of the screen in 16:9 mode, but with a darkened room it never seemed necessary.
With my current projector - the zoom lever is on the front right above my head, and it only takes a second to zoom out or in depending on the content.