MyFA > Pack and Ship > scroll down > Create Shipping Hold Locations > New
From there, you can choose parent/child or single locations. All single hold locations start with "SHP Z" and you can type any letter followed by two numbers for a hold location. To replace the cart locations, it makes the most sense to stick with the same format already being used: 1 letter for each cart, followed by a number for each tier in the cart. So you would type something like ZA01, then ZA02 and ZA03 and put one on each tier in place of the old ones. Print duplicates for the other side of he cart, remove the old parent locations labels, and repeat on each cart.
The single hold locations locations were intended to be used for flats or tubs, and have the benefit of being able to add multiple batches, even after scanning it in to the pack station. This was really helpful before they added the "release cart" option and you had to call the CSC for every "hold location in use" error, but I still like it for the flexibility.