Is the Weber lid too big or too small for the outside diameter of the drum? If it is too big you can try fiberglass rope that is used for fireplaces and wood stoves. My local Ace hardware carried it for about $7/ for 12'.
An easier route would be RTV. Any autoparts store and probably even Walmart carries RTV. It is silicone gasket in a squeeze tube and good for up to 500*. It costs roughly $6 or $7 a tube. Simply line your lid where it meets the drum with RTV and give it 24 hours to set up (usually less but I don't how much of a gap you need to fill). Once it has set up, put the lid back on and check for leaks. You also could get a better fit with the RTV by putting a strip of wax paper over the fresh RTV and then putting the lid back on the drum. That would allow the RTV to conform to the drum without sticking to it.
If your lid is too small to fit the outside diameter of the drum as my Weber knock off was, I used a pair of channel locks to bend the vertical surface of the outside of the lid into a horizontal surface. My drum had a welded lid so the outside diameter was a little wider than I anticipated. You will have some initial gaps but it is nothing a 4 lbs sledge, a little patience, and some RTV can't fix.