The Oruch HaShulchan O Ch 43 (13) writes that one can put ones tefillin on the floor if they are in a bag which has a חלל טפח. He refers to מגן אברהם סעיף קטן י"ג וסעיף קטן י"ד .
צריך דווקא כלי שיהיה בו חלל טפח, דאז חשיב אוהל להפסיק בינם להקרקע
You need specifically a vessel that has a חלל טפח since then it is considered an אוהל to interpose between the tefilin and the ground.
How do we understand the vessel with a חלל טפח?
Is it sufficient that the vessel has a space of a tefach or must it have a space of a tefach in addition to the space taken up by the tefillin?
related: Placing Tefillin on the floor