Originally Posted by warriorofmight

Rule #1 NEVER copy the default folder into a custom folder. The way EQ works is it loads the selected interface upon startup. If only a couple of pieces are in that folder, EQ automatically loads the rest from default, so there is no need to add any of those files into a custom folder. Doing so will cause the UI to "break" with future updates.

To answer the question, the file you're looking for is EQUI_CastSpellWnd.

Odds are the window will not look exactly like it does in the Sparxx UI. You can try copying over Templates or Animations, or try editing the file yourself to change the appearance.