Using the paper pattern technique above, get is close to the right fit. Then place it in the trailer and scoot it as close to the back as possible. Then take a compass (for drawing circles, point on one end and pencil on the other) and hold it with the point against the rear wall keeping it (the compass) parallel to the length of the trailer. Follow the rear will with the point and the pencil will make an exact copy of the wall offset onto your luan. You can then trim this to a perfect fit. It's the same technique used for fitting counter tops to walls in homes.
Using the luan for pattern making is a good idea, too.