I have a Kingsford Grill, which was difficult to smoke with, due to massive air gaps. The lid has about an 1/8th inch gap all the way around. The ash tray has a 1/2 inch gap along the back side. Even with all 4 vents closed, it would cruise along at 350 until the coals were consumed. It is a decent grill, but smoking required temperature management via controlling the number of lit coals, thankfully it has a front access door for the firebox. Unfortunately the fire access box also leaks air.
I bought a 15 foot roll, of half-inch wide, 1/8th inch thick Lava Lock. The lid surfaces were lined after cleaning the mating surface with rubbing alcohol. A double thick gasket was made to help seal the ash tray as well. For the time being I left the leaky firebox door alone; I wanted to see how much difference was seen with the changes made.
On one of my mini-WSM builds, the lid had some side-to-side play. It got lava lock all the way around the inside of the lid, to eliminate the side-to-side play.
Another of the smokers, a WSM-14.5 has a well-known gap at the bottom of the Side-Access door. A couple short pieces of lava lock, which were tapered to prevent gaps were used to seal the bottom of the door.
Today the Kingsford was fired up, and to my pleasant surprise, temperature could be adjusted by the vents. A temperature of 225 could be set and held. Charcoals actually lasted 5 hours, rather than the usual 1-2 hours. I think getting the fire door sealed, will actually improve it more. The grill was started with an entire chimney of Kingsford blue (rather than my usual minion method for low temperature cooks). This way to SWMBO, it looked like I was grilling, when in actuality I was nowhere near grilling temperatures.
SWMBO sometimes feels like I spend too much time smoking; this allows me to smoke, with the "appearance" of grilling. She loved the chicken and steaks which came off the "grill" today. Chuckle....