Self-amplified spontaneous emission (SASE) XFELs are unique sources producing extremely powerful, ultra-short X-ray pulses. However, they rely on the amplification of the electron beam shot-noise, resulting in a relatively broad, spiky radiation spectrum. The poor temporal coherence of the SASE radiation is very unfavorable for many applications. A self-seeding scheme using X rays from the first part of the undulator to seed the second part, based on diamond single crystal monochromator, allows producing nearly Fourier-transform-limited X-ray pulses with the 1e-4 relative bandwidth at 9 keV. In our presentation, we will report on the main features of the custom designed monochromator system capable of handling the severe heat load coming from the high repetition rate of the FEL pulses (up to thousands pulses in trains with 10 Hz repetition rate) as well as the spontaneous radiation. High-quality synthetic diamonds used in the seeding monochromator combine high lattice perfection, low absorption of the x rays, very high heat conductivity and small thermal expansion. For deformation-free mounting of the diamond crystals to the holder, the crystal plates are manufactured in a special form with cuts that decouple the mechanical fixture part in the holder from the defect free working area. A hard X-ray self-seeding monochromator setup will be ready for installation at the SASE2 undulator beamline of the European XFEL soon, and its commissioning is planned for the beginning of 2019.