Brilliant idea to wrap the soft container around the Steripen to protect it.
I kicked all kinds of things around and you will find a bunch of threads buried here with cut down Platypus containers, beheaded Smart Water bottles, and so on. I swear that every time someone found a trick recycled drink bottle that would fit a particular model of Steripen, the drink manufacturer would change the design.
You could use a beheaded quart milk jug, which is flexible for scooping, handy in camp, can store the Steripen along with soft goods, etc. Quart sized plastic mayonnaise jars work great too and are 2.2oz with the lid (great for storing dry grains and the like as well).
I liked using the HDPE Nalgene bottles which are a couple ounces lighter than the transparent Triton or polycarbonate ones. I've always wondered why Steripen doesn't make a mating PET bottle or a top to fit the large Nalgene threads.
Many people use their cook pot, which wins on several levels: no extra stuff, multiple use, no problems stirring, etc.