So, to expand on the idea outlined earlier here's a couple of sketchbook pages to illustrate what I'm talking about.

The above sketch shows the wide open backscene typical of forest country in new England perhaps

This is exactly the same layout sketch with the backscene changed to something more like the Midwest .

The scribble below each layout concept illustrates how the sloping backscene would look in profile. I'd bolt the backscene onto the back with a few coach bolts and wing nuts. There shouldn't be too much of an issue with the tree backscene. Perhaps the extra depth of the prairie might be an issue but this only needs to be made from the lightest thinnest ply so it wouldn't weigh too much and throw things off balance.

As another note I checked my stock of baseboards and I have a frame ready made that is 54" x 14" Almost the perfect size.