@Paknaloid Here is an overview of how I modded the rear. First of all, no change to roof and B & C pilars. As for the rear glass, if you are familiar with the front windshield design of the 10248 Ferrari, I used a similar design...with the glass mounted on hinges, and it simply leaning against the roof line. I then attached the decklid to the rear glass in the same manner as the base instructions. This glass & decklid assembly are attached to an all new mount point of 4 (#2434) parts (that are built up on a 2x8 brick on the back of the trunk floor).
Also, I widened the decklid by 1 each side, and replaced smooth plates to be rounded plates at end. Finally, I built a simple horizontal surface outboard of the decklid, that is mounted on a angle plate (#21712).
I may have oversimplified some of this, but hopefully this (and images) point you in the right direction.