Wtf dude, your plug is coming loose. Get a different one. Anything. Whatever.
You said
It wouldn't preload my headset tight enough and after a few rides it would loosen up, which obviously isn't any fun to ride around with a rattly loose headset.
Now you're saying the headset is fine it's just the plug that's rattling?
I have pretty good reading comprehension and I cannot figure out what your issue is.
If your headset is properly adjusted, it does not "come loose" bc of the plug. If it comes loose it was not installed/adjusted properly - unless the problem is being caused by something other than the plug.
Unless you are saying that the crapppy plug is preventing you from installing correctly. In which case it's not loosening after a few rides, it was loose to begin with.
Move this to the mechanic forum.