The 8x4 layout (I'd have said 4x8 myself) is based on a 'standard' sheet of plywood from your local hardware store or lumber yard. Buy a sheet, maybe run some stripwood along the edges to stiffen it up, and drop your oval down. You can easily fit 18" safely, and 22" can just be barely accommodated.
In speaking of imaginary locos, what might an American-market English Electric 'road' loco have looked like? Tall cab behind a 37-style nose, with slightly wide hood?