Reading through this thread gave me the idea to take a typical recycled bottled water container and just cut the top off for use with the Steripen. Cheap enough and light. It could be used for stowing the Steripen, spare batteries, Caldera Cone, cooking utensils, etc.
With a little grocery store research, I'll bet you can find one container slightly large in diameter than another. Cut the top off the larger one and slip the smaller bottle inside for stashing in your side pocket. Dunk the big bottle in your water source, purify and decant into the small-necked bottle for storage and drinking. A one quart milk jug with the top lopped off would do the trick.
FYI, I've been using the HDPE Nalgene bottles which are a couple ounces lighter than the Tritan/polycarbonate ones and a couple dollars cheaper too. They make a 48oz "Silo" model that is taller and the same diameter and cap.