Two letters, working together. One from the U.S. employer describing the professional tasks and the period, and one establishing that a professional in the relevant discipline will supervise the work, because that supervision is written into this profession's own entry.
An unusual entry with an unusual proof
Scientific technician or technologist is the odd entry on the list. It does not ask for a degree at all. It asks instead for theoretical knowledge of a specific discipline and the ability to solve practical problems in it, and it contemplates work in support of a professional in that discipline.
That structure changes what the file has to contain. Hypothetical example: a technologist near Saint-Joachim-de-Shefford who calibrates and services soil-moisture and frost-sensor arrays is offered work by an American research station. The employer's letter describes the instrumentation work well, but says nothing about who supervises it, and the file has a hole in exactly the place this entry looks.
The fix is not more pages. It is a short, signed statement naming the supervising professional, their discipline and their qualification, plus a description of how the technologist's work supports that person's programme. Keep the theoretical grounding visible too, through transcripts or training records in the named discipline.
Vague words such as specialist or field technician help nobody, because the officer is matching the file against one printed entry.