A Hammond diecast aluminum project box. A 4 pole double throw switch. Speaker binding posts of your choice (6 pairs are needed).
The diecast aluminum is soft enough that it can be drilled easily enough with a hand drill and standard bits, and it takes spray paint well. I made one with with a 8"x6" (I think most of the boxes they make are 4" tall, which is just right for pairs of binding posts) and that was the right size for me. I made a smaller one and that one got bullied around by the weight of the cables, and there was not enough room for all the connectors.
I like a big beefy toggle switch on the top side of the box. And since this is handling the power from the amps, you want a high amperage rated one ( Mine is 20A I think, and this is also where the beefiness comes from. not to mention the contact area will also be much larger than lower amp rated switches) If you cant figure out how to wire it, use your multi-meter to figure out what goes where. You might think this would be the most expensive part of the project but its not. (a quick search gave me a 15A switch for $5)
Then all you need is to install the binding posts, and solder in the wire (again this is fairly high power so use heavy wire) I like 4 pairs on one side of the box (for you, for each pair of speakers) and 2 pairs on the other ( for you the amp)
I have two of these in use in my system. I have a AV receiver for movies (with satellite speakers) and a Bottlehead system for music. I switch the L/R channel speakers to which ever system I am using. The other is a much smaller one for the sub-woofer. Since this is line level and not amp output level, I used a small switch, and obviously RCA connectors instad of binding posts.
