I like this analogy:
Imagine a massive wall of lockers. Each locker is 1mm by 1mm, and the entire wall of lockers is a square 2 light years on each side. When you choose a private key, you pick one of these lockers at random. When someone sends you bitcoins, there's some magical inbox which puts the bitcoins into your locker without telling the sender anything about the location of your locker.
I like your analogy, but I think your math is off (or was it an estimate as a figure of speech)?
2 lightyear equals
1.892 * 1019 mm. A square wall gives
3.58 * 1038 lockers.
There are 2
160 different Bitcoin addresses, which equals
1.46 * 1048 possibilities.
As large as your locker wall is, you'll need 4 billion of those walls!
Allow me to create another analogy: imagine using quite large cubes as lockers, 1.2 meter on each sides. Now, build a
cube instead of a wall, and it will be 1 lightyear on each side.
Try to imagine the size: 1 lightyear is
63240 times larger than the distance from the earth to the sun.