Custom Report looking for differences between records

Custom Report looking for differences between records

I'm having a hard time figuring out how to create this report. 

Let's say I have these forms.
  • Product
    • ID
    • Name
  • Package Template
    • ID
    • Template_Subform
      • Product
  • Package
    • ID
    • Package_Template
    • Package_Subform
      • Product


There are other fields in the Subforms, but they're not important. I would like to find out which packages don't match the template. So it's something like:

Show me all packages where Package.product not in Package.Package_Template.Product

Thanks for any help.