The cabin door opened in cruise flight and the private pilot subsequently lost control of the twin-engine airplane. The airplane entered a descending right turn, and when ground impact was imminent, the pilot shut down the engines and feathered the propellers. The airplane then impacted the ground in a field. According to FAA inspectors, the door would not lock due to improperly installed upholstery. The pilot/owner reported he did not get a certificated mechanic to inspect and sign off on the upholstery modification.