Numinix Product Variants Inventory Manager for Zen Cart
[3.6.13]

This mod was exactly what I was looking for as I couldn't get 'Stock by Attribute' to work with the newest update of ZenCart. I also love the added features- like the Dynamic Attributes compared to 'Stock by Attribute'. I wish there was a bulk feature but other than that small flaw it works 100%, kind of a pain to do each product one at a time and as long as you don't have too many attributes it goes pretty quick. They installed this mod without issue, and I will use them to install future mods.