Some grill doors provide access to the cooking grate. These doors are common on barbecue smokers and need to be tight enough to keep smoke inside the grill. Other doors are there simply to contain heat for faster cooking. Another type of homemade grill door opens into the charcoal bed, allowing you to add coals while you cook or to remove spent coal. This door doesn't need to seal as tightly as doors to the cooking compartment.
You can seal grill doors around their frame and edges. A metal seal is the sturdiest option, whether it's a crimped metal edge or a separate piece of metal that you attach once the doors are mounted to create a complete seal. Fiberglass seals, like those that keep fireplace doors sealed, are another option. If your grill has two doors, overlap the seals where they meet, and designate one door as the door that must be opened first and closed last to make the center seal line up.
Even if your homemade grill door seals tightly around its edges, it might still leak heat or smoke elsewhere. For example, a grill door with a glass window could have leaks from the seams around the glass. To seal this part of a door, use a heat-resistant silicone sealant. This type of sealant won't crack or expand when exposed to the grill's cooking heat. It also won't release any dangerous chemicals when heated, which makes it safe to use around food.
A grill door won't seal tightly without a latch, which holds the door in position. If the latch fails, replace it before adding material to your doors to make them seal, since this will make it harder to add a latch in the future. In addition, make sure your doors are constructed from heat-resistant materials such as metal and tempered glass. Materials that expand due to heat will warp and lose their seals quickly.