Why do you guys even care? Telling someone, who you don't know from Jack, that they have no right hauling a trailer if they can't do it a certain way, in no way adds to this thread. I'm not arguing that it's not the way this sort of thing is taught. It's also not how I do it. I honestly couldn't care less what methods someone on a forum uses to back up a trailer. It literally has zero impact on me. The OP came here asking for help, and help was given (by a few).
I've used the seat belt extender method for years. Like
@LouM mentioned, I just keep the extender plugged in and unbuckle from the extender. One of the reasons I've driven with the door open/seat belt off is when driving a new fence line and dropping off fence poles at intervals. Getting in and out for each one is a pain if I'm having to put on and take off the seat belt for each one. With the door open it's a lot easier to keep the tires where they belong when I can look down and see the ground directly while creeping forward, and is especially helpful when navigating ditches.