At the time, I had no idea how to make one for LARPing. A previous attempt failed miserably.
Recently, I was perusing a tutorial for Amtgard throwing weapons when I saw this photo.
It dawned on me that the third weapon from the left would make a decent, simplified representation of a plumbata. The head can represent the arrow tip and the pommel is akin to the feathers. I think we can ignore the lead weight in the middle.
I decided to give it a try.
Here is the result.
By the way, here's the link to the tutorial.