GBZ Buttons
From sudomod
A/B/X/Y
The A, B, X, and Y buttons of the GBZ can be any combination of NES, GB, and SNES buttons. SNES buttons will need simple modification, removing the smallest tab.
D-pad
The D-pad can be replaced with the NES controller's D-pad. The SNES D-pad is too large to fit.
SNES
L and R trigger buttons can also be used, see chiz's GB Pi2 build.