To tame a rex make either a 3x3 or a 5x5(5x5 has more room to trap it) then 4 doorways high all the way around it. Next just build ramps that go to the top of the trap. Now all you need to do is have it follow you up the ramp and fall in as you jump out the doorway. :)
If you use the bear trap strategy which is if you lead it into a Dino gateway that is shaped in a triangle and with a bear trap in the middle (Large Beartrap) and when it is stuck you must put a dinogate behind it to trap it and then knock out.
The Rex are very slow . Put a stone gates around him while he is stuck and use a bear trap then put the x3-x4 stone gates then tranq him out . Make sure you clear the area before setting the bear traps others creature can spring it and u would be left with using your self or a tanky dino for a distraction