Nobody ever thinks of this as a "snack", but a big glass of milk can hit most of your requirements. Fast, cheap, high fat (if you choose), high protein, calcium, leaves you feeling full. When I'm really in a hurry (like, at the airport), I'll chug a big milk and be good for ages. Plus, I'm guessing if you're making a lot of milk, milk (albeit from a different species) might be a wise thing to get more of.