When I was looking for a headliner board for my square body crewcab, when no one was replicating them, I had the thought of spray foam. I was going to use some of the thicker painters plastic, like a 7-9 mil and get it set down on the inside of the roof, get a diy spray foam kit such as the frothpac, and spray that on there, then carve/sand it to the shape I was wanting. Of course there is one big problem with that, it has to be upside down... So either get another wagon roof and cut it off and turn it upside down for a template, or put your body on a rotisserie, but that's not easy either. The great thing with that setup though, is it would be relatively simple to have molded in speakers or lights that would look otherwise "factory", rather than something added on.
I should add, I ran out of ambition and money on that project and it got parted out because of life getting in the way, so I never actually got it done...