Thanks for the pics. They prompt some questions.
Will you seal the edges of that waferboard, if not the surface. Newer product isn't as vulnerable to moisture, but the underbody is a harsh environment.
For an example,
BamZipPow's Dark Aero He started with dimensional wood, but gradually replaced it all as it deteriorated.
Will the boat tail lift with the tailgate? Will it underlap the bumper to prevent a step abaft the diffuser?