just bag them up . get some fish bags from your lfs , they should give you them free. they should also be able to hook you up with a polybox. bag the fish up plave the bags in the polybox and pack newspaper around them so they dont slosh around too much . they will be fine like this.
for just 5 hours you can probably get away with not using heat packs. stuff plenty of newspaper around the bags and the lid on the polybox should be fine . 5 hours isnt long at all. the car will be warm and polybox is a good insuklator