How to mark paper documents before scanning to simplify OCR sorting?

My business generates a lot of handwritten notes that get scanned and sorted into clients’ files. Each file has a unique 5-digit numeric code, which is handwritten on the page before scanning. Our handwriting is inconsistent enough that OCR doesn’t work well for identifying the file number .

Is there a clever way to quickly manually label a paper document with information that will be easily recognized after scanning? We can’t pre-print a barcode ahead of time, and it needs to be simple for office staff to do.

I’m looking for a simple way for several non-technical people to quickly add information by hand to facilitate the automatic sorting of scans.

We can pre-print the pages with additional graphics, if it would help the OCR process. For example, we could print something like a Scantron form in a specific part of the page.

Is there an easy solution for this?