A grease drain is a luxury. I ran for 10 years without one and just put a cookie sheet in the bottom of the chamber. (For the record, my wife was not happy her cookie sheets were always missing.) My point.....weld it shut as heat is more important than grease.
I looked and measured that darn hole thinking about what I could find to plug it up but yet maybe make so it could still be used if wanted, I came up with all kinds of ideas and when i was spaced out i thought you stupid idiot you have a metal lathe setting there and all kinds of aluminum round stock just turn one down with a cap on the end and drop it in place.