Okay, I found a design off a pamplet I have. Its the northwest miniaturehorse club obstacle handbook. Wish I had a scanner.
But here's what they say for instructions: 1- 4'x8' flat base reinforced plywood (looks like it has a frame with a plywood on top) and 1- small 4.5 round log to make bridge rock. Lay bridge over log at the center point.
I design the courses and want to make one. I think that if you design the bridge with a V indent in the side panels and then place it on a V (upside down) log, than it would rock. Haven't done it yet though.