Have you tried filling the original plunger head with hot glue? and while the glue is still how, it will make the plunger head comform to the plunger tube, and while drying, it will make a somewhat perfect seal? I don't really know semming as I have never tried it. I just thought of it.
The only problem with that is that the crossbow plunger tube gets narrow as it gets closer to the end. Also, trying to take out that hot glue would be a pain.