No one will make you custom tubs, tubs unlike racks need to go through molding process, that mean create a mold and than get the plastic and use the mold to create the tubs. No one will do that for 1 customer needing tubs once.

When people build their own racks or have some made custom they use tubs that already exist either ARS, Freedom Breeder, Vision or Sterilite or Rubbermaid.