From what you have written, I'm under the impression your F9F is very nose heavy.

The easiest fix is to move the battery packs aft but if that's not possible, then adding ballast is the only option. You might want to consider using rosin core solder that comes on a spool as it's very flexible and easily shaped to fit anywhere. The least noticeable location would be inside the duct, attached to the inner circumference, as far aft as possible.