I purchased a Vibroplex Vari Speed in order to slow down my Bug. However, it did not slow it down enough for what I wanted.
So, I added a home brew method to slow it down even further.
See pictures below.
What I did was take some model airplane fuel line about 2" long and add it to the end of the pendulum.
Then I took a piece of model airplane control rod and stuck it in the fuel line to make it rigid (you could use a small nail also).
Next I started adding 9/16" nuts to the end of the fuel line until I got it as slow as I wanted.
I left the vari speed on the bug so I could also vari the speed.
I have a range of about 10WPM to 18WPM. You can play with the length of the fuel line and different size nuts to get what you want.
Looks ugly, but it works.