Roof plug 25.

Went for removing rear metal panel completely
figure i can get enough strength from rear wing, and vent , to be
added below that. Options seem to be a shelfed vent, or just rounded in line with back of car.
Decided to wait for airflow test. It seems its possible to make your own smoke pellets using Potassium Nitrate , sugar and i believe Baking powder, to slow the smoke process down. Also Dye can be added , which might be a possibility worth considering.