From Jim Adams:
I made a plug from balsa and finished it to a smooth finish second I used two aluminum pans (the kind that you throw away) I filled the first one with plaster took the bulb (well waxed) and placed it in the plaster half way in I used two pins through the center to hold it down. let that dry then pull out the plug next put thin saran wrap over the mold and place the plug back in the hole.
Now comes the fun part I used rubber bands to hold the plug in place (remember I had two pins that extend past the mold walls) next fill the second pan with plaster and lay the first on top it is kind of messy but it works. When the second half dries (about two hours) pull them apart. You will need to plug the holes at the ends on the sides and create small air path upward in the and a spur (looks like a funnel when you
are dune this needs to be big enough to pour in the led) at the end.