You can choose any type of container for a PSK, its what works for you. It can be an otterbox or Altoids tin. I chose the otterbox because of its waterproofness. Basically you need something small enough to carry on your person and that will protect your contents. An otterbox roughly cost around 9 buck where an Altoids tin cost around 2 bucks.
Another reason I chose the otterbox is because its clear and I can easily identify my contents. Its virtually indestructible.
Take time to develop your kit, do it once and do it right. Then always keep it on your person.