I would suggest using aiborne express. I have used them 6 or 7 times and never had a problem.
As far as packaging I would not worry about heating for an overnight trip. There is no amount of heating that you could provide that would work for a trip like this with tortoises this large. Heat packs are small and used with small tortoises shipped during a cold season. Your big guys should do fine for an overnight trip. Ship the tortoises at the same time but in seperate containers. I would go ahead and build stout crates for the tortoises with solid walls and 1 inch holes for air. The crates can be built for a tight fit and be sure to cut out or attach some sort of handle for the shippers to lift it by.
This will increase your cost of shipping but it has to be done. Rubbermaid containers would work, but you run a risk of the containers cracking and breaking during transit. Also the tortoises WILL defecate and urinate during the trip and wood will absord most of the wetness while a rubbermaid would contain it well, but they would be swimming in it. The crate would be the best route in my opinion.
Good Luck,
Tortuga