Completed! Case is 8″ by 6″, 2.75″ deep at back, with sloping front. Has true bypass. Green light lit when not bypassed, red light lit when set to “dirty”. It has a hum. I'll have to debug that, but it's usable now. Most of the hum stays even in true bypass, so I suspect it is the wiring around the 3PDT switches (12 VAC used for lamps).
I will be adding a DC supply for the lamps and filaments to try to kill the hum. The circuit and PC board for that is below. There's not much space left in the box, so it will have to be on a small PC board to fit in.
Partial BOM (I had many of the parts already)
A schematic of my modification will be posted later