Hi Jason,
Efficiency wise, the invertor is probably the worst option for you because of conversion losses, and, starter batteries don't like being discharged very far before being recharged. You could risk damage to the battery's capacity to hold a charge if you don't start the car to top it up regularly.
Does the portable radio have a DC input? If so you can find a 12 volt cigarette lighter adapter to run it at Radio Shack for about twenty bucks. It could also likely double as a charger for your Ham gear just by selecting the right voltage and swapping the end(I'm assuming it's just a 2meter handheld).
If the radio doesn't have DC in you could get a 12 volt charger for rechargeable batteries. Energizer makes one that will charge four hicap AA NiMH's in 15 minutes. Occasional use of that would be no strain on your car's battery. Well, unless you're out for a month without starting the car
Good luck, and hey, nice, tidy panel work in your avatar
-Greg VE7GJM
Give a person a fish and you feed them for a day; teach a person to use the Internet and they won't bother you for weeks.