The way I like to store my milkcaps is in storage sleaves, designed
for photo slides. This way I can flick through all my milkcaps in a folder and
it's easy to see them all and easy to see which ones you are missing as well.
You can get these from your local photo shop. I use Jessops
or Snappy Snaps.
Alternatively you can use old toilet rolls with paper or a card disc taped over one end. Make sure you use toilet roll tubes that are slightly bigger than a milkcap or you might have trouble getting your caps back out.
There are also a lot of proper 'pogtainers' that you can use. Some pics are below: