Back in the old days, I used to step the mast on my Hobie 16 by myself all the time. I would tie a line to the end of the forestay, run it through the shackle or around the bridle itself, and back to the front crossbeam. After I stepped the mast, I'd reach down and grab the line on the front crossbeam, pull out the slack, and tie it off there or cleat it off. Then, go foreward and pin the forestay to the bridle. Done. This was in the early 80's, but I don't think the set up has changed much over the years.

Good luck.