You could use almost anything as the base material for your trigger. I am using wood because there's lots of free scraps at the shop at my school. Wood also provides a good resonance, good stick action and is pretty durable. You could use a big hunk of wood like the one I found above, or 2x4s would work really well too.
For the electronics, you only need two parts: (1) a piezo transducer
& (2) a 1/4" phone jack
, both of which you can get for a few bucks at Radio Shack. You can probably order them online somewhere for cheaper too, or just pull them off of old electronics.
A piezo element is a sensor that measures force and converts it into an electric charge. That charge become the signal, sent through the phone jack, which is sent to the drum machine or interface.
Tools you will need include: bandsaw or handsaw, drill press, drill bits, boring drill bit, soldering iron, epoxy glue, screw driver and ruler.