My way of dealing with the non-gun-friendly states I have to drive through between Florida and New Hampshire is to over-comply with FOPA. Every gun is in a locked case, whether it is a pistol or a long gun. All of the ammo is in a separate locked case. The idea is to be in such strong compliance that there isn't even a question about it. It is also a good idea to keep a copy of the law with you, in case the police are not familiar with it.
I should also point out that none of this should matter if you are not stupid. A traffic stop is not probable cause for a vehicle search. If you drive reasonable and act in a mature, professional way if you get pulled over, there is no reason why this should even come up. If the police ask if you have weapons in the car, simply inform them that you are not required to answer that question. If they decide that your "evasiveness" is probable cause for a search, then you cooperate fully, show them the law and show them how you are in full compliance with the law. If they arrest you at that point, they are completely screwed in court. The search having been illegal and the arrest having followed a clear demonstration of your compliance with the law.