Turning a Pet Travel Carrier into a Crate
Many customers add an expandable portion to the pet carrier to give the pet more room while keeping the size and its use within the Airline's requirements. Some use the expandable carrier as an overnight crate.
As the expandable portions do not usually have any support structure, it should be used when the carrier is on the floor.  :Otherwise, the pet could be sliding to a side making the carrier in balance and uncomfortable for the pet and person carrying it.