I made the button box from a Ps3 wired controller and soldered the switches to the pcb and the handbrake lever is connected via a switch to the box. Also I rigged a led light to the frame for night races.
Awesome Button Box!
I made mine the same way, soldering directly to the contact points on a PS3 controller.
Great idea for the light for night races too! I have a USB slot on my rig
to which I plug in one of those 'laptop keyboard lights' for the same reason.
I also bought one of those key switches, but never ended up installing it as I didn't want to put it on my button box and couldn't figure out a way of mounting it (easily, anyhow) on my cockpit without it spinning when I turned the key. I was going to connect mine to the wheel-side of the AC adapter box, so that when I turned the key (with the USB connected) it would spin the wheel, (I guess as close as you can get to turning the car on lol). What do you have it hooked up to?