After the catastrophic results of our last attempt to send samples on dry ice, we went back and did research on packaging materials for ice. We learned that we need to use foam and strings as plastic tend to be brittle. We also checked various tubes to see how they hold up in cold temperature.
We finally selected tubes that were less brittle and better packaged in their box. Only the larger version was available, and so we ended using tubes that were much larger than needed, but we decided that this would not harm the content.
Assaf re-run the whole experiment again. This time it went smoothly and he got samples without any probelms (so we do learn from experience).
This time each box was padded both inside and outside, and securely wrapped in foam.
We then packaged them in the box, and wedged them into place with pieces of styrofoam. We broke the big blocks of dry ice to small piece to ensure that there is no big hard mass bumping around during shipping. Since we didn't have a large box, we sent the samples in two separate boxes.
To our relief, the two boxes arrived safe and sound.