I did my dual battery mod on 6/7/08.
I have had the switch for some time. My father bought it for the motorhome, but never used it. I finally found the perfect use for it.
I would like to have installed twin Optimas, but funds are low.

I cut , hammered drilled and welded the new battery tray from a single piece of sheet steel.
I dimpled two of the bolt holes with a ball peen hammer to keep the bolt heads away from the battery's shell. All edges were smoothed to prevent chafing of nearby harnesses.
I added a ridge made of rod to the tray to allow for the use of a GM battery clamp set-up. I was not able to find the plastic block used on GMs, so I picked up a conventional tie-down kit and used it instead.

The tray is bolted in to allow access to the harness that runs underneath it.

Last, but not least, the switch controls the GROUND connections instead of the positive connections. I actually read the instructions and diagrams, threw them aside and did it my way.
It is so much safer to run ground cables all over the place and keep positive cables short and direct.
The alternator wire still runs to battery one, as always. Battery 1 has a positive jumper to battery 2, so the alternator is essentially connected to both at all times. However, only the battery with a ground connection will charge. It may seem backwards, but think about it; it still breaks the circuit.

Here are the pics: