This going to be about 2 mods done at once.
1. Mustang 3rd Brake light Relocation. Instructions found here
https://www.ft86club.com/forums/showthread.php?t=72225
The only thing I did diffrent was create my own shelf cover from coroplast and carpet. - Saved .5lbs I also think it looks better nice and smooth
2. Carbon FIber plate to fill rear seat hole. This was done on the TRD Griffon although it was hard to find exactly what they did. THis only added .5 lbs over my coroplast/carpet. This however should add structral rigidity to that hole which is the main purporse plus I think it looks Sick!
So the Shelf first

weights

Stock shelf weight
Had to relocate the keyless entry computer from off the rear shelf
NOW onto the CF PLate this took the most time to cut and get right

OLD plate and some of the plastic removed that was no longer needed
Used rivnuts for the top bolts (TItanium BOlts)! rivets for the bottom
I will post more pics later today with the rest of the interior installed. THinking of making a diffrent plate for the bottom seat area.