I used the GP CG Machine. I rested the pads on the monokote and it worked fine. It left a little indentation in the covering but after an hour it disappeared and no need to even hit it w/heat gun.

As for accuracy..I'd say much more accurate than the finger balancing method and right up there with the best methods I've seen. Simple physics dictate the CG machine..small ball joint for low friction and tiny fulcrum. I could move my battery as little as 1/4" and watch the balance change...very accurate. Just make sure the posts just touch the leading edge and are square all around.