How about an "over-bottle", such as a Nalgene bottle of a size to hold the Polar Pure bottle? One of the cylindrical, wide-mouth versions, maybe even in Lexan. See their web site at: http://www.nalgene-outdoor.com/store. Look at the Jars page. (Similar in function to the "over-drums" used to hold leaking 55 gallon drums & contain any spills.)

It would add some bulk, but might be worth it.

David