Suggestions on keeping rats frozen?

Repticon is coming up and I'd like to get a bunch of frozen rats while I'm there. The only problem is that I live over an hour away from the event and do not own an ice chest.
The last time I drove home with frozen rats they almost completely defrosted by time I got home, even though they were covered with ice.
Does anyone have any suggestions on how I could keep them completely frozen during the trip home?
I was thinking about putting them in some kind of container with dry ice, but I don't know.
Any suggestions?

put them in a plastic bag w/ dry ice, tie it up and put that in a styrofoam box or the same bag in a box with home insulation or i guess bubblewrap wrapped in aluminum foil.

also wear layers and don't turn on the heat in your car (or keep it low).

Go to wallmart. Get a styrofoam cooler. Get 2 x 5 lb bags of ice and some newspaper. Stick the ice below and above and wrap the rodents like crazy in newspaper put it between ice and stick it in your trunk. You will be surprised how cold that keeps it. Should be less than $15 for the whole thing and easier than trying to find dry ice. This is of course if you can't find dry ice. If you do have dry ice then get that and wrap it in tons of newspaper and stick that in a cooler in your trunk.

If no dry ice is available...

Block ice lasts longer, so I'd say get some 20oz soda bottles, full with water (leaving some room for expansion) and freeze in advance. Or else buy the gel-ice packs. Also, pre-chill the cooler, so that the ice/frozen rats don't expend energy on cooling down the cooler for you. A fully packed cooler also works better than one with large open areas.

Lot of people up here in deer territory swear by ice packing in the Coleman Xtreme coolers for long hauls. They use bigger ones, but the 28qt version should work just fine for what you need.

Add further insulation with those mylar space blankets. Plenty of people have those up here too, for winter breakdowns.

What you can do is pick up the cooler beforehand and get ice near the repticon to eliminate it from melting partially on the way to repticon ;)

Styrofoam beer coolers are pretty cheap and will hold a lot of ice and rodents.

Thank you for the replies! They helped a lot, and will hopefully help other people too.

Ice is not much good at keeping things frozen. Rats will thaw at some temperature below 32 degrees because of the electrolytes (salt etc) in their body fluids. This means that your rats will thaw before the ice does. I know this from years of transporting frozen fish. Dri ice is the only way to go. It is readily available in super markets in my area.