Check out this website:
http://www.mardar.us/It has free printables, but they are decorated. You could print what you wanted, then use it as a template. It's a little more work than just getting a "ready-to-go-template", but it's the best I could come up with.
If you click on "Boxes and Bags", there is a snack bag template on page 2 (scroll down and keep scrolling!), which looks like a popcorn box with angled sides and no top... You'd need to make 2 copies and glue them together, but it looks easy enough (I'd try it first on typing paper rather than waste good cardstock, tho')!
Don't forget to post pics of your boxes when you're done--would love to see them!
HTH!
Popcorn Box Template? *merged*
