RJ4 - Your idea makes sense to me as we don't use the sink either. However, one would need to get different faucets to control the shower. My Model 211-S has the exact same shower stall as yours. "cheeze1" has a model 213 that has the stool separate from the tub/shower. It does not look like the dimensions are right to fit our space. You can see the back side of the shower sink by looking in the storage place for the shore power cord.
Our Shower -

Your Floor Plan -

cheeze1 Floor Plan -

Years ago I did know a fellow who made molds for fiberglassing by making a mock-up out of cardboard and spraying the outside. He painted the outside of the cardboard and it seemed to leave the paint on the inside of the fiberglass and the cardboard merely peeled out. Would need a lot of patience to line up with the floor drain and stool drain. Good Luck.