Don't create mockup based on content. You'll never get it on time, and if you do, there are sure to be changes. create a mockup base on structure. Allow some wiggle room.
The word "content" is too broad to rely on. For example.. don't ask for content for the shopping cart. ask for a list of info types that need to be included ( and preferably their higherarchy or importance )
so you can have a info block:
You can always arrange the other / and remove items logically from there. Hope that helps.