I saw directions once for painting on a T Shirt. It said to put a sheet of cardboard, wrapped in foil, INSIDE the shirt and smooth it over it and then pin carefully, being sure not to stretch the fabric.
Seems to me that might be the first step in putting on the Wonder Under or whatever you use with the picture or whatever it is you want on it. Make sure it is in the correct place before ironing. Then when cool you can do the sewing.
Think this will help?
And thanks for the idea of using a smaller sized dohickey for the front of the T Shirt. I have some around here that I can do that with. Never thought of it. I think I want to put 3 pictures on it that write out my name!!! The third one will be a bird in flight.