My Triple A has the shorter megaphone, so this might not work for you, but I used an extra long bolt in the muffler support mount, and catches the end of the center stand. The load isn't on the megaphone, since it's bolted to the frame there too. I'll snap a pic and post back.
Regarding the other forms of stop, the idea that I've always seen (and what was originally employed on my F2) is a stop bolted to one of the swingarm pivot pinch clamps. The stop is a wide flat tab that hits the top part of the centerstand, bent to adjust the height of the centerstand. This is an easy solution, but make sure to use a substantial piece of metal for the task. Whoever made the one for my F2 used steel strapping, and it deflected over time. A friend's F1 has a nice tab made out of 1/4 inch steel.
And, not to over think such a simple modification, but the other key to getting the tab method to work well is to get sufficient torque on that pivot pinch clamp. I add a spacer in between the clamp ends to limit deflection, which seems to work well. (I think I used/modified an ignition coil spacer/standoff) Just make the spacer short enough so that the "clamp" still squeezes down on the pivot pin. If you just bolt the tab to the clamp, the tab will move around, requiring occasional (and frustrating) adjustments. No good in my book.
