I used silver adhesive vinyl used on cars for those decals you see on back windscreens for example of hotted up cars saying ‘TOS’ or ‘No Fat Chicks’.. uh anyway.. It looks great and helps find the zones.
I had zones marked out for the top corners for back and forward in web browsers and the scroll zone. I tried setting the bottom area for other actions but they where always getting in the way.
I’m thinking even a strip of tape just wide enough to feel the delineation would be adequate. Guaranteed, his mod will end up being the nicer looking one