Here is what my billboard will look like.
I found a few nice examples of images that fit in that space, and I also found one that sort of fit, but not quite.
That's a great sign, with a great slogan, but when I copied it into my billboard, it left a little too much white space on either side. (There's some white space on the billboard above, too, but somehow, it looked wrong to me on my ground-level billboard.)
Now the lazy way to fix this problem is to simply distort the image from side to side... which I never do. When you do that, it makes faces look too wide, it ruins the typography, and it's just the wrong thing to do.
What I did was: I cut all the elements out and made new layers of them. Then I made a pale yellow backdrop of the correct dimensions, and I moved and resized all the elements to fit in the new space.
Now, I'm not going to name and shame the HO scale sign manufacturer who doesn't do this (I suppose any reasonably clever reader could Google around and figure out who it is) but here's an example of how NOT to do it.
Take your existing circus poster...
And then squash it down to fit your billboard.
Just look what happens. All the lettering is fat and squished, and the gorilla looks too squat. Why anyone would think this looks authentically vintage is beyond me -- it's so obviously distorted by a modern graphics program. And the company selling these says they're for circus savvy customers.
The moral to all this is: resize, don't distort!