i would suggest stick with the Teensy....
which Teensy are you using?
this one would be the best option (Teensy 2.0)...
as trying to get a controller to fit is just way extra work (especially for space management)
getting the analogs to work shouldnt be a major issue...
you could always swap out the analogs on the Wii U for the ones that are on the controller....
but, thats just way too much work...
are you sure you have a genuine Teensy 2.0 (not sure you can even get a clone of it)
make sure you have programmed/loaded the script onto the Teensy properly
if you have applied the wrong script or would like to reflash the Teensy...
you would need to reset the device and put it into program mode (this normally comes up when you connect to you PC)
if you facing issues...
try deleting and reinstalling the Arduino software
make sure you have the correct drivers installed
make sure your using the correct script
.