I like to put mine at the back of the bed butted up against the tail gate, so I can have the dog jump up on the tail gate then enter the kennel then I shut the kennel door then the tailgate. The tailgate covers the front of the kennel so it's a tad more security so the kennel door can't get opened and let the dog out.
I don't like reaching over the side of the bed trying to shut the door up front, nor do I like climbing in the bed of the truck all the time. I started doing this when I'd go up a dirt road then stop at every locked gate I came across then walk in hunting grouse = a lot of stopping and going.
Only downside is if you're going down rough road the dog can get bounced around a little more at the back so I slow it down over the rough big stuff so the dogs head don't bounce off the roof of the kennel.