Certainly you can sandwhich the piezo between the neck & bridge.
My preferred location is on the 3rd (or 4th) string side of the neck under the lid, below the bridge.
I attach the disk using silicon to give a bit of a sound cushioning/dampening.
Good luck with the build
thanks for the reply. and that is what I was thinking. just hallow out the neck a little so it sits I there flush and glue it on top And bottom with some silicone glue and then glue the neck to the lid.
Yep, carve out the neck a little so there's clearance between it and the soundboard. Brings two advantages - (1) the soundboard is freer to vibrate and should give a better general sound (2) you can fit a piezo on the underside below the bridge. Don't take off lots of wood - just enough to give a clear gap.
Try to avoid having the piezo in contact with the neck or you'll risk picking up lots of finger noise.