Made of "thick plate", "thin plate" and "HV kapton".
The thick plate is a precision machined FR4 tray that provides: positioning of the straws, mounting of the module to external support structure, gas menifold for distribution of ionization gas to individual straws, and cooling tube feed through for module cooling.
The thin plate, bond to the inner surface of the thick plate, provides high voltage connections to the straws.
The HV Kapton, bond to the back side of the thick plate, brings HV lines from outside of module to the thin plate and straws.