Looks like a half inch hole.
you could measure the size of of it and also the pitch of the thread to know exactly the size you need any plumbing store should have but its prolly easier to take the part off and just bring it with you.
Appreciate the input, I'll hit up ace, maybe even the plumbing co in my area to see if they have one. There's no taking the part off, unless I take the whole pvc pipe off the main drain.
if you still have the nut cut a piece of plastif from anything. maybe milk carton or something flat. put water proof glue or maybe plumbers putty on it to cover hole then more on inside. screw nut on and check for leak.
I don't and I was thinking of some ghetto solutions until I found that ABS nut, imagination was running wild.