Thrust tube should be a straight line from the rear of the fan to the outlet, preferably about 10% smaller at the outlet. This model swells out after the fan, then reduces down to the tail. This is a big no,no. Intake should be about 10% larger than the inlet area of the fan too. These are just rule of thumbs, or is that rules of thumb.
Also ducts should be smooth as possible. Those little molding dimples aren't doing any good either. Anything to keep the airflow as smooth as possible is a help.My first ducted fan was in 1955 with an English freeflight Vernon kit. Looked like a MIG 15 but different mfg. I remember now, it was a Lavochkin or LA 15. All stringers and covered with silkspan paper. Fan was bent up of aluminum with an Allyn Fury 15 glow for power.