Whoa, that looks amazing!

And I think that the 1/4" top is more than enough. I agree with Simon it needs to allow vibrations through to the microphone on the other side. But since this is all new to me, I don't know for certain.

I should get the amp stuff wired up in the next few days to test the electrical side of things.

FrankenWashie, I see you are building your own. Do you want me to build a piezo preamp in a box to send up to you? Or do you have something suitable already?