The ideal wood for the DIY slate?
I'm not really sure there is an ideal wood.  In fact, it really doesn't even need to be wood.

The entire point of a clapper is simply to make an audible sound that comes from an easily visible source.

As long as the sound is loud, short and crisp you are fine.  If you wanted, you could have your three year old be the clapper instead and just give him two pots to bang together.

The convention of a clapper is simply because it is a convenient device that also allows for indicating other meta data about the shot.  The clapper part is just included so that you can also capture the audio sync data without needing another device.