I've never owned a Plumb Bob before, and never really had a need for one, but I was at our local Iron Monger the other day and saw this piece of Hex Brass Rod 1¼" across the flats, and it spoke to me.
I turned a recess to inset the wood piece, I didn't get a picture of it but I drilled and tapped a hole in the center.
Turned it around and tapered it.
I turned a cap the same way but I forgot to take a picture of it.
I soldered a eyelet into a 5/16 brass bolt to hold everything together
added a piece of Rosewood and screwed it together
flushed it all up, I haven't located my buffing compound, when I do I'll polish it. It's about 5" long.
And it's plumb!