Thanks AJ, a little too late as I order the one from Amazon. It doesn't have the threads, but the larger plug actually fits in the hole of the reservoir pretty tight. I was able to leave it on with the engine running and it didn't move at all.
Sadly, I have a leak somewhere else. I am guessing at this point that it may be the water pump. I had a slow leak since around 30k miles, I had to refill around every 6 months or more. Now I have to top it off almost daily. It looked as if it was the lower radiator output, but I replaced the plastic adapter with a new one Saturday and the leak continues. Only that side of the radiator gets wet, I don't see any leak anywhere else. I tried to look from underneath the car when I had it on the lift and didn't see any white residue near the pump. Sucks!
At this point I guess I may take it to a radiator shop or the dealer to diagnose, I received a letter for the water pump about a year ago so hopefully the dealer will honor that and fix it. I doubt it as I am almost at 90k now.