I see you have tried the monokote idea, how'd it work out? The 1/16" ply can either be finished with a couple coats of epoxy or 1-2 layers of light cloth, lots of people just epoxy and seem to have good results. One thing to bear in mind with the 4 strokes is they don't have the same response as the 2 strokes and are heavier. One thing that I've been tinkering with is a self contained engine pod that can be easily transferred from one hull to another, I'll add a photo of my idea.
When gluing the sheets of foam together I usually have a good idea where the cuts will end up and avoid putting glue in that location so it can be easily shaped after. If your cuts end up where the glue has been placed the glued area doesn't sand as easily as the clean area and you may end up with a deformity if you're not careful.
