!!
Try a cork until you get the right plug. Go to a local craft store or maybe HD and get a bag of assorted sizes for a buck or so. After you cram it in, you might have to get it out w/ a cork screw.
Why this suggestion? The service that closed my pool one year somehow removed a plug from my heater & stripped the threaded socket in the process. No reason for them to remove it in the first place and now the plug won't screw back in. Can't isolate the heater - filter to heater to pool only - and water is pouring out the hole. Anxious to get the filter running, I grabbed a bag of corks I had and gave one a try. I, of course, intended to promptly do a righteous fix, but now, 5 years later, the cork is holding fine.
Good luck!