This is very much based on the Craig Anderton design found in the book "Electronic Projects for Musicians". Pretty gong show ring mod ;).
I was doing them with a probe for a while but all that is left is 1 in black as pictured below for $50.
The pots control effect volume, clean volume and modulation frequency. The Small toggle switch turns on and off a low pass filter. Again the video shows a unit with the probe, but the black one available now does not have the probe, otherwise it is identical.
Here are some photos of past builds.