Saw
that today so I thought I'd give it a go.
It's quite fun to use, it could maybe be useful for slowly introducing a solo or a riff without having your hand on the volume pot.
I used a 1458 for Ic, 5088 for transistors and a J201 for the fet, I think it will work with anything though.
Forgot to mark the switch, it's a spst momentary that connects to ground.
Schematics and more info can be found on the link above.
Made and verified :)