You can buy from places like KMART a slotted/compartmentalised storage box with a hinged lid for around $20 - I think they are Hot Wheels brand and are blue. They are built specifically for matchbox/hot wheels cars.

Just went and counted - they fit 48 cars and are called a Hot Wheels Carry Case, they look really nice and are a good way to store the cars. You could get a couple and stack them on top of each other.