The thicker the inflated polystyrene the better.I would use about 5 pounds but simple to try filling the box from one of your recent deliveries and timing it if you have time to do this

I don't really think you can do it per weight of your sample, because the biggest influence is the external temperatures and transit methods.  However, for 24 hours I believe you will be fine if you just bury your samples in dry ice.  It sounds like the boxes are adequately insulated.  I just suggest you don't overload them with samples, and make sure you can fit dry ice around them on all sides.

Thanks all....definitely going to pack 2 inch thick dry ice bricks around all of the samples and on top.  I plan to pack blankets around coolers to prevent complete sublimation. They will be in a U haul truck (ie, cold outside from Illinois to Texas)....

You can't fully seal the edges of the box when using dry ice.  You effectively create a bomb.  I would highly recommend not sealing the internal box.

I plan to transport these with without tightly sealing the foam containers...the blankets would be to help buffer the temperature and would not be tightly wrapped...I suppose I should keep the back of the U haul cracked or crack the roller door every time I stop for gas (probably every 2-3 hours) to allow the escape of gases.  Thoughts on that? I will also crack the windows in the cab of the truck to be on safe side although I think it is sufficiently separated from the storage area in the back (it is a box truck 10 feet long).

Everything went great!  I drove with both windows cracked in the cab of truck and stopped for gas every 3-4 hours...when I stopped I opened the back of the truck to vent it.  Samples were nice and frozen when I arrived!  Thanks for the suggestions!