You don’t need a trap to tame these, there are less costly taming methods.

1-This only works for low levels but if you line up a bunch of large best traps and bait a Rex into it it is a pretty easy tame.

2-If you use a creature you can shoot off of you can just follow the Rex around shooting it until it is knocked out.

3-If you get on to a hill or big rock that the Rex cannot get on to you can just shoot it from there until torpor run then chase it with a relatively fast creature to finish knocking it out.

I have never used a trap but from what I have seen and experienced using a trap is much easier than any of the methods listed above.👍