Good solution! I started thinking about this same thing when I got my steripen. I came up with a slightly different solution.

Have you seen these new disposable plastic containers with screw-on lids? Ziploc calls them Twist 'n Loc containers I think (I use a store brand of the same thing). They are transparent but fairly durable. They have marks at 1 cup, 2 cups, etc. So I can use that to measure my water and stir with my steripen. But even better than that I found that the 4 cup size is exactly the size to hold a Heineken pot. I put my fuel, liter, dishtowel, and windscreen in the pot and put the pot in the container and screw on the lid and it holds it all tight so it doesn't rattle. I use it mostly as a cosy. I put my freezerbag filled with boiling water and my dinner into the container and screw on the lid. It keeps the heat in and saves my food if the zip closure on the bag fails. When it's done I open the freezerbag and fold the edges over the rim of the container and it's my bowl as well.

If I need to use the Steripen I just take the pot out. Fill the container with 4 cups of water (about a liter) and stir. Then I pour it into my Platy or whatever.