Securing a spring line to a winch barrel where there is no handy cleat is best achieved by the tugboat hitch. This is also the favoured method anywhere a single post or bollard must accept a line that may need to be released under serious load.
Take three or more clockwise turns around the winch. These carry all the weight. Now lead a bight (a loop) under the standing part (the part under tension leading off the winch barrel) and lay it over the winch. Pass the end back around the winch and you have enough friction to hold most loads. If the Massive Maru is on the other end, repeat the hitching process one more time to make absolutely sure. There is no knot to lock up, and to release it under load, you have only to keep a hand on the turns as you lift off the parts of the hitch, then surge away carefully just as you would a genoa sheet.