I was at the Outback Cup this past weekend and looked at the Columbia Sailing Club signal barge. They have a very simple and effective barging buoy mounted on the transom. I didn't take a picture, but I'll try to describe it.

It's basically just two pieces of PVC pipe tied to the transom to form a tall isosceles triangle. A small hippity-hop is tied to the vertex. A third line (also attached to the vertex) allows the structure to be raised (for storage) or lowered so that it sticks straight out behind the barge.

Now, Lake Murray doesn't get much current, so the barge wouldn't tend to lay perpendicular to the wind, but such a fixture could be rigged to stick out from a signal boat in pretty much any direction, provided you have tie-points.

I hope that helps,
Eric