Have you tried thermal blankets? Insulate the cooking chamber from the outside.
Has extended times noticeably by reduction of thermal energy loss.
The idea came from of all things... horse blakets.
Lang 84 Deluxe Kitchen known as "The Glamazon" , Custom Klose Backyard Chef... of Snot, and a 22.5 WSM
Lang 60 Mobile - The Damsel II, Lang 48 Patio - The Damsel, Bubba Keg Grill - RIP, Double Barrel Smoker and a BSKD