Added
MATERIAL_GUI_FILLER("WHITE_STAINED_GLASS_PANE"),
MATERIAL_BUTTON_UP("LIGHTNING_ROD"),
MATERIAL_BUTTON_RIGHT("LIGHTNING_ROD"),
MATERIAL_BUTTON_LEFT("LIGHTNING_ROD"),
MATERIAL_BUTTON_DOWN("LIGHTNING_ROD"),
MATERIAL_BUTTON_UNDO("AXOLOTL_BUCKET"),
MATERIAL_BUTTON_UNDO_USED("BUCKET"),
MATERIAL_GUI_PLAYER("PLAYER_HEAD")

for even more GUI configuration options!
(Recommended to not alter the MATERIAL_GUI_PLAYER )

ps also added a readme to the github