To effectively catch most of the grease, I think you'll need a large pan that is at least half the diameter of the barrel.
I've seen some very large stainless steel bowls sold that might do the trick.
Or you could try the "brinkman water pan" from the WSM-like brinkman smoker. See here for details.
Lots of the WSM-folk use a large clay pot base or pizza stone with or without the pan as well.
This gets into the idea of a heat sink in the cooker, which some people like.
Something like sand or ceramic briquettes (about 1-2 pounds worth) in the pan give it a heat-sink function which might stabilize temps over long cooks.
Whatever pan you use, be sure to cover the top with foil for easier cleanup.
Of course, using a pan will make your cooking "indirect" for better or for worse, and you won't get that sizzling vaporized/burnt fat flavor.