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This box is a nice way to house a switch, two relays and a dozen of speaker jacks (connectors) on the back. Box is 160x60x100 mm so it can fit somewhere neatly.
Holes for plugs are 7.7 mm in diameter, some 17 mm spaced (both A and B input), and one group is 22 mm apart, to be used as output R and L pairs. Front switch is 12 mm diameter (push switch in my case, but toggle is also nice). you can get a bigger one, as it could be easier to operate (go for 14 mm or 16 mm if you want). I took one with a backlight.
I suggest you use a relay(s) that can switch both negative and positive plugs, as today one can expect to have an amp that is in bridge mode or simply that negative terminals are not grounded.
I have used 5A (AC) relays with 5V (100mA) coils, so they can be easily powered over the USB. However, there is a 7.7 mm round hole at the back, just snug for a barrel power connector. I did use a USB-A connector on the other side of the cable. Of course, If you are careful and use a mains switch, theoretically you can fit small power supply inside, but I advise against it.
Have fun with it!
