I currently have an invoice, where some numbers are written by hand, and I would need to read them automatically.

My past experience is that people who write numbers will them write in so many different ways (overlapping with filed name etc.) that it's really hard to isolate only digits.

Now I would like to redesign invoice, for example, to add filed of some rectangular shape, that will be easily detectable (think easy to detect like a QR code is).

 I'm not sure:

 should I add some ordinary box, or are there any specific designs of fields/boxes/rectangles that would make finding the field and isolating everything in it (from image) simple and 99% correct?

Tech stack that would be used is:

 - OpenCV
 - Python
 - HTML/CSS for creating the design of invoice, that would be exported to pdf for users to fill it with the needed information