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Not being able to see this, but going by "drain plug" and "under"...
I'd also vote for JBWeld. Thoroughly clean and degrease a large area around the hole, and the threads, to make sure it binds well. But since you may want to unscrew a drain plug at some point in the future, I'd go a bit further.
Either drill and tap a new smaller drain plug in the middle of the existing one, or JBWeld a plate over the area around the hole, and (again) drill and tap a new drain plug fitting into the middle of the plate.
So either way, JBWeld, but providing for a new drain plug of some type before you seal it up.