Best practice to index just a few folders within a very large folder

Hi All,
there is a folder with many many folders in it - I try to just index some of these folders, e. g. only those, containing a particular text in the filename. How to do this avoiding adding all of them one by one by hand? could a finder smart folder used for that? (containing only those folders matching a query and then index that smart folder).
Thank you in advance,

The folder’s name contains this string or one of the files inside the folder?

the folder’s name

Does the desired set of folders to be indexed regularly change? If that’s the case then the only idea coming to my mind would be to use a scheduled smart rule performing a script. And this script would be have to look for the desired folders, check what’s already indexed and add the not yet indexed folders to the database.