Happ and a few other places used to sell button labels eons ago. They were clear plastic washers that went under the button and had a clear tab with text printed on it (A, B, START, ECT). If the gap isn't super big those might work. I don't have a clue if they still sell them or what they are called to search unfortunately.
Of course the other option is just to go to lowes/home depot and get some plexi.