Robert,
Your comments about getting good mechanical strength on the upper portion of the tail extension caused me to decide to beef up the mounting system dramatically.
This also means that the extension will not be used for trunk space, as you shall see in later photos that access to the "trunk" will be limited due to the beefy mounting system.
This first picture shows one way of getting more mechanical strength from thin sheet metal. The Insight is designed to be lightweight, and this also applies the sheet metal in the rear of the car.
Unfortunately, this also means that attempting to mount a lever arm off the back of the car means possible trouble if not done carefully.
The panels are thin enough that they easily "oil-can" when pushed in various locations. Because of this, any place there are transitions in the sheet metal, the ribbing causes the panel to be stronger in that location. We have to utilize this feature when mounting strengtheners to the car.
To this end, the back of the Insight is going to be beefed up to allow adequate mounting strength for the tail extension.
Here is another view of a way to strengthen the sheet metal. Any place were there are creases in the sheet metal, this is also an area where lumber can be used to aid in the stiffness of the assembly.
This shows the reverse side of the mounting lumber, and shows how smooth the finish is here. This was done by mixing thick epoxy and adding to the back side of the lumber and attaching it to the car and allowing to setup. Grease was used as a mold release, and even then, some areas stuck and had to be carefully pried apart after the epoxy setup.
This shows reinforcements being added to this "tab", which will later be used as one of the main structures to hold the tail extension.
This picture shows the embedded 5/8 inch fastener that was epoxied in place for good strength. This 5/8 threaded rod was drilled and tapped with a 1/4-20 thread for later attaching to a 5 inch long bolt.
Jim.