  A Set of new MPGD-based Photon Detectors is being built for the upgrade of
COMPASS RICH-1. The detectors cover a total active area of 1.4 m$^2$ and are
based on a hybrid architecture consisting of two THGEM layers and a Micromegas.
A CsI film on one THGEM acts as a reflective photocathode. The characteristics
of the detector, the production of the components and their validation tests
are described in detail.
