Its not actually a filler 'cap', its the filler ring/seal that fits tightly around the cup but it does have a lip that fits over the edge of the cup. On the sides of this ring there are 'cut-out' areas on the filler ring/seal for the area where the wick goes through clup slots and into the cup....It's important as a properly seated filler ring prevents leakage into that cup and down the airstem...
I just put mine back in upside down. I do use cuticle nippers to make a larger center hole for easier draw.