I bought a Silver Bullet Jacket for my 22.5 WSM and it's great. I smoked an 11 lb. Berkshire pork butt last week and it was a 17 1/2 burn without adding more charcoal and I had about 1/4 of the original charcoal left. Between the DigiQ and the jacket, I've got a winner.
The Silver Bullet Jacket is made of high heat silicone that's good to 2500 degrees. It has a small slit at the bottom to fit around the draft inducer. I seal all the dampers on the bottom and leave the one on the lid open about 1/4. When I smoked the pork butt, the outside temp was in the 50's, rose to the 70's, then back to the 50's. I put the jacket on at the start and never took it off. Temp stayed constant through out. Can't beat a DigiQ.