Piggyback rack 

The Piggyback Rack is the foam security and storage system that lends a new level of ease to handling microcentrifuge tubes and test tubes, eliminating problems when tubes fall out. Once inserted into a Piggyback Rack, tubes can be easily lifted and stored to free up block racks for long term experimental procedures. The Piggyback Rack will not absorb liquids and is resistant to most solvents, acids and alkalis. They can be marked with a ball point pen or marker on their convenient writing border.