check out the possum pride website
http://possumpridetrikebodies.com/ they have numerous fiberglass components available and it appears to me that some of the molds are close to (if not identical to) the wilmac bodies. They show the rear trunk and the seat insert that you seek, and with a phone call to verify fitment you may have an affordable source for your needs.
If needed, i have available a digital copy of the wilmac assembly manual. Pm me with a direct email (large file so i won't send via the triketalk system) if you want me to send you a copy. No charge and it comes with a money-back guarantee!
Good luck with your new project.