You can easily see what happens when you leave something on top of ground goop that hasn't thoroughly dried. But it ended up being no problem. It did eventually dry and - bonus! - I had some nice indications where things needed to go.
The masonite was cut wide enough to provide a flat area for both the street and sidewalks.
And in the photo above, you can see the street mockup laid over the masonite. Also, note the track covering in preparation of yet more ground goop application.
I used some goop to fill in under the masonite road base and pressed the masonite in place, keeping it level.
I also applied another layer of goop to the surrounding area to bring everything up to the same level.
I also applied goop to he right (north) side of the masonite, again to blend everything in.
And, finally, a few shots of the overall area after all the goop was applied:
Lastly, and admittedly not a great shot (you'll have to click on it to enlarge), you can see the masonite road base is all nice and level:
Unfortunately, it wouldn't stay that way. More-experienced folks may have already guessed what happened next. But the rest of you will have the "fun" of discovering that later - as I did . . .