OK! Back to business of shaping the tank plug. It's a real challenge trying to keep an amorphous glob of foam relatively square, so I used an old piece of glass from a sliding armoire and rubber glued some rough sand paper to it to help square up the major "facets"
I then roughed out two pieces of cardboard from some drawings and pinned them to the side to help establish the profile. I ran out of foam mix towards the edges there but hopefully it won't matter since I'm rounding the edges once I'm ready for shaping the contours.