Labor in inventory assemblies

I am setting up inventory item codes and assemblies for a new client who makes tree syrups.  I've decided that in order to capture all the costs related to harvesting the raw material (ie syrup), I'll create a GL account called WIP then move the balance to the raw material item code once I know the total amount (gallons) harvested.  Only then will I be able to calculate a unit cost for my raw material.

My problem is how to account for labor costs that go into assemblies.  I now want to create a build for a bottle we sell.  So I need an assembly with x amount of raw material, a bottle, a cap, a label (all easy to identify and cost) as well as labor to make this assembly.  I don't know what the labor historically costs per item so I'm not sure how to add it or IF I can add it.  Suggestions for how to make this work are appreciated.  COGS will not be correct unless I can add labor to the products.