question about drum and Weber kettle lid

Its possible the lid will just on top of the drum with the lid removed. Some use the locking ring to hold lid in place. Odd fitting lids may require you to remove the top 4 inches of the kettle and attach it to the drum as an adapter. My drum was just a hair too big so I removed the rolled edge and sanded the edge smooth.

Typically the 22 1/2 Weber lid is a very close fit to the top of a 55 gal. drum. Although it seems from readings here that most Weber lids need some persuasion to get the exact right fit. If you have the opportuniy it's best to take the lid to the drum source and find a drum that it fits best.
Based on my limited experience it's important to have an air tight lid to help control temps.
You will likely need to do some bending on the Weberlid to get it to fit the way you want.
I never really could get mine to fit exactly right so installed some latch clamps to help which worked pretty good. If nothing else having the clamps on get the lid in the same position each time and keeps FIL out of cooking!

There is a latch on both sides.

SFB. what are those clamps called? my lid is just a tad too big and I have to wrap foil around the drum at the seam to seal it off. I need a fix.

There just basic latch clamps I bought off Ebay. I had to make some small wire loops to make it work with the lid. May not be optimum but serves the purpose.