Aircraft technician reported helicopter engine cowling came loose inflight and damaged rotor blades.

Narrative

I performed various inspections on the aircraft; two of which required me to open the right hand hydraulic cowling door. I first performed an AD; followed by a visual of the hydraulic deck area and cleaning of the hydraulic deck area in accordance with the 30day airworthiness inspection. After the inspection and cleaning; the hydraulic cowl was shut and latches secured; I then moved onto the rest of the inspections. Prior to returning the aircraft to service; I had the pilot come out and visually inspect the engine chip plug installations; after that the transmission and engine cowlings were closed and a walk around was performed. The aircraft was then returned to service. After the aircraft had flown 1.6 hours; the Right hand hydraulic cowling came open in flight while the aircraft we on final to its home base; this resulted in the four main rotor blades striking the aft corner of the cowling. Aircraft safely landed and various inspections were performed; a new cowling was installed; blades repaired; and aircraft was returned back to service. During further investigation; it was noted that not all five of the fasteners were secured after maintenance was performed. It was also noted that this was not caught after three different walk arounds of the aircraft.