Company letterhead and signed by the Master is all I’ve ever used, except for a few of the USCG Discharge ticket thingies. Oh yeah, and the ship’s stamp if they have one. A description of what you did on the ship, where it sailed, gross tonnage all certainly will help the evaluator.
Anyways, anything under 100 tons is approved for self-certification so you just write it up yourself and go. This is how many fishing/hunting/yacht “Captains” pop up, out of the blue, and hang a shingle in your town.