Contemporaneous internal records: design and configuration documents, change histories, authored training material, project files, and any registered intellectual property, together with the standard relationship and payroll evidence.
Prefer records nobody wrote for the petition
Evidence created in the ordinary course of business is persuasive precisely because it was not made for this purpose. Configuration files with the employee's name in the revision history, an internal manual they wrote, a project record showing the sequence of work on a proprietary system, and a purchase file showing the equipment was modified rather than bought off the shelf all speak for themselves. Registered intellectual property helps where it exists.
A small operation may genuinely have little written material, and the honest response is to gather what exists, describe the record-keeping limits accurately, and let a reviewer weigh it. Overstating thin documentation is a worse outcome than presenting it with its limits acknowledged.