It's very easy to make American cheese at home, and it happens to make the very best macaroni and cheese. As you say, mix some other cheese with sodium citrate dissolved in water. Cheddar works great. You'll get a nacho-sauce-like goop that you can pour onto your pasta (cavatappi or fusilli are best). Add in a caramelized onion and you'll never want to eat boxed mac & cheese again.