If the 7-button bank is intended for the right hand with a stick in the left hand for Defender/Stargate, don't forget the Invisio button on Stargate which is one more button than Defender. Depending on how you arrange your 7 buttons, the button bank can be perfect for Stargate (and Defender as well, just don't use the 7th button).
Also, as someone already mentioned I think, if the button bank is set close enough to the stick that you can hit the 7th button with your LEFT thumb (using your stick hand), then that can be Defender's reverse button. But then you still need the proper number of buttons in your right hand, including that 7th one, perhaps using the right side (Player 2) button bank, for Stargate.
I wonder if what I just typed makes any sense...

It's hard to describe control panel layouts in words.
Eric.