A hand full of packing peanuts does exactly the same thing as the expanding foam and is a lot easier and less messy. Bubble wrap folded up and placed inside works well too.
I've tried the expanding foam. It works, but is messy and takes a few days to dry. It will keep expanding for 2-3 days, and you can't take it out if you change your mind later.
Why would you change your mind about filling a hollow stock?
