I used the Microsoft Keyboard Layout Creator to customize my Spanish keyboard to the English OS because the standard Spanish keyboard didn't quite match my physical keyboard.
While I got it (thanks to MKLC) to 100% match my physical keyboard, I had also added other useful characters mapped to roman characters to make it easier to type (rather than remembering ALT codes). Unfortunately, those Alt Gr extra mappings only work in the MKLC test window but do not work when the layout is deployed, installed and configured as input method.
For a better description with figures please look at my post at the Microsoft Answers subsite.