The drain is in the lower pan, which is pretty easy to remove. I'd pull it and drill and tap for a tapered pipe plug - I think 1/4" would work fine. Get a brass plug! That won't seize in the steel pan. The tap will be $5 or $10 (and you'll have it forever), the plug maybe a buck or two. You'll need a pan gasket one way or the other though you could just use RTV. Whatever way you go, use something (I use a "tweaker" small straight screwdriver) to clean out the socket head screws that secure the pan. If you round out the hex in one of those you'll curse yourself. Somehow I bent a screw and a trip to the hardware turned up a 6mmx17 socket head screw in the little cabinets - no issue.

The only reason you need to pull the pan to do this is to make sure there's no filings in there. If you use grease on the drill and tap you could do it without even pulling the pan down - depends on your comfort level. BTW - if you don't have lots of drill bits, Lowe's in our area is selling a 2-pack with the tap and the bit and they're not very expensive.

Good luck!

Dan