How to make ctrl-p plugin for vim only search within a given list of directories

Question

I am working in a mammoth codebase. There are too many files and folders at the top-level. But, I work in a small corner of the codebase - no more than 10 directories hanging off of the top-level. How do I give ctrl-p the list of these 10 directories and have it search only inside them? Right now, even with let g:ctrlp_max_files=0, ctrl-p is having trouble finding my files because I just have too many files.