Setting Custom Attributes in AdWords Auto Targeting

I have a Google Merchant Center product feed of about 350 individual products, most of which we resell and are not authorized to advertise via Paid Search. I have about 90 software products and 12 hardware products that we currently offer that I WOULD like to include in Product Listing Ads on AdWords. Unfortunately, the attribute options are very limited and all are far too general. I want to have one Ad Group that Targets ONE product, for example.

How can I do this? Can I somehow set a custom attribute for item number, or should I use the adwords_groupings or adwords_labels attribute and modify my data feed to include one of those two columns, and just set each attribute individually for the one’s I want to include? What do you think? I have been looking everywhere for an answer on this.