Not sure yet. I was thinking featuring one of the official card backs (doesn't matter which of the three), and then you have artistic license with anything that wouldn't scare off the stereotypical American male (which means no colors such a male would consider feminine, such as pink and brighter shades of violet), since I anticipate most of the potential participants wouldn't exactly be comfortable winning a pink emblem.
Oh, and I suppose the event it's for would be a good thing, too. That would be "Pokemon TCG GT8 Winner - Heat x", where "x" is a number between 1 and 3 (I tentatively am having 3 signup deadlines, but may not need to end up having three). You could remove the "- Heat X" portion, though. I suppose one emblem for all three "heats" would suffice quite well.