There are different ways to occupy a wider projector screen using a projector with a small aspect ratio.
The first option is to zoom out the image on the projector. In this case, you can zoom in on a 21:9 movie image to cover a screen that is wider than the 21:9 aspect ratio. In this case, there will be a spill of light outside the screen frame. It can be sometimes difficult to notice since the spill is usually from the black bars of the source.
An anamorphic lens is also another suitable option for filling a wider screen. In this case, you will adjust the projector to get a vertical image stretch. Once this is complete, you can stretch the image in the horizontal direction by placing a lens in the projector path. This will produce an image with a normal appearance.
The use of an additional lens will result in a lower contrast ratio when using the anamorphic lens.