Box: Map value into Starting Folder field

I am attempting to complete a proof of concept demo for a client that requires that certain file attachments downloaded from Salesforce are uploaded to the appropriate folder in a box folder hierarchy.

Each opportunity gets a top level folder with subject related folders inside of the opportunity folder. 

Finding the opportunity level folder seems straight forward because the starting folder is static. I haven't figured out how to get the ID of the subject level folder within the opportunity folder. I assumed that I could first find the top level folder and then use the results of that search as the starting point for searching for lower level folders for the correct folder to file the attachment to.

When doing a Box file/folder search, how do I set the starting folder based on a prior search within the recipe? Is there another way to achieve the same result?