White laminate "Formica" or other brand cut to fit will work. I usually can find enough in a trash can at a cabinet shop to do the white stuff. Keep that key, it is valuable. Interesting to note that Tournament Soccer used the same locking system on their first tables. If you can get ahold of the original Life magazine through archives or Playmeter or other industry pulblications there are photos of the Patterson machine that changed everything. I grew up in pool hall that had one of these for the price of two dimes. We would get a nickle apiece and play. I first saw this table at the age of 18 months while my Uncle Kenny supervised his busness, The Recreation Inn AKA "the hole" becaus it was in a basement of a large historic buiding in Downtown Rapid City, South Dakota. The year was 1959. The high school kids would put me on a chair and we would play. I was too little to play pool, but I cannot remember ever not playing foos. 48 years ago I started and I'm still having a blast with this crazy game. I know your kids will too.
Happy Foosing,
Oly