My experience is that the following method works well and it is easier to move stuff around to balance the airplane.

You might try hanging the airplane from above with a mason twine cradle from engine to tail. Then drop a plumb bob from the suspension point. When the plumb bob stops swinging it will point to the balance point. The airplane should be approximately level or at flight attitude.

I lay the battery (or weight) at various locations on top of the airplane to find the best balance location. There is nothing to buy or make and the wheels can be a few inches off the floor or table just in case something slips. The point of attachment of the cradle to the airplane is not critical.

Bill S