I forgot to mention, in the last post, that the horses were assembled months ago. I cut out the horses and then traced the outline of one horse onto 4MM foam board, cut out the blank horses and wrapped the 2D horses around the foam board. Nothing fancy, but the foam was easier to work with than 4MM cardboard. (I did use a knife for foam board)
I assembled the standard coach box, with recessed windows, and then glued the cut-outs onto the structure. Photos attached show the result. It ain't beautiful, but it did turn out better than I expected.