Hi Mark,
I don't know if you still want to draw a layout for this pedal but 've used the schematics from FSB to build it and it works!
http://s1297.photobucket.com/user/AlexLondon9/media/render_zps48c9235b.gif.htmlI think an external Gain pot is much more useful than an internal trimmer.
I've tried your version with the switches instead of the single rotary one and it does work too but I find the original version easier to use.