Wish I would have seen this before. I burned mine out from the outside in with a weed torch, it was a peice of cake. 3 gallons of propane and about an hour is all it took. The phenolic on the inside literally just fell off when I hit it with the torch from the outside. I could give it all the heat in the world from the inside, but once I started burning it from the outside in, that resin coating just fell off.
Good luck on your build. I used a nibbler to do all my doors and sheet metal work. You might want to look into renting or borrowing one. You can cut a door in less than a couple minutes.