First off, do you have a closed or open top? Basically, did yours come with a removeable top or did you have to cut the top out. There a several ways to go about mounting a weber lid to each style of drum.
Mine is a closed top. Carefully grinded lip until the top fell into barrel. Then ground outside of lip, stopping every so often to test fit lid. Got mine to fit pretty nice. Once you cook on it a few times the seasoning will help it seal. If you decided to go this way, I'll describe more in detail.
Btw, if you do decide to do any grinding, use a soft or "floppy" disk. Not the harder, thicker kind. I learned that the hard way. I ruined the lip on an open top. Hard disk wants to bite in to the lip and gouge. Soft disk conforms more to the radius. :)
UDS, mini wsm, 22.5" weber kettle