Started out in the '80s with a Suburban...
I started out in the 80's with a Suburban with two of the plastic airline dog crates in the back. Hunted ducks and pheasant with two Labs. It worked but as has been mentioned, it can get really stinky in there. Driving back home for 6+ hours with the aroma of stinking marsh mud really isn't that pleasant.
On top of that, the two dog crates side by side at the rear of the vehicle blocked easy access to a lot of the storage room. I was always sliding kennels in and out to get to my gear.
In the 90s I went to Ford pickups with cappers. My hunting buddy and I figured we'd do a longitudinal kennel that took up just half of the bed, all on one side. It made it real easy to get to gear on the open side of the pickup bed.
The attached pics show the present rig. Still using that original kennel. It's generic corn cribbing which is strong enough that we tie down big Rubbermaid storage containers on top of it using bungee cords when making the long drive to SD. We welded up a simple door that has held up for 20+ years. I have a half gallon bucket of water strapped to the inside which we refill as needed from a 5 gallon jerry can. The jerry can fits exactly between the kennel door and closed tailgate. With the tailgate locked, you can't open the kennel. That's all by design. The bottom has the restaurant rubber pads with drain holes to let the water go through and run out the tailgate after swims. We usually put some old rugs or carpets in there when making the long drives to keep it soft and comfy.
It's a dark capper but we've never had heat problems even though we have hunted some pretty darn hot opening weeks. Capper has a front window and side windows and when it's hot we open them all and leave the rear window fully open.
There's a 70 pound black lab in the kennel in the picture. We have actually had 5 Labs in there at once when moving between fields (very short periods). That's pretty crowded but three fit in there very easily and four is not bad. They all can lay down but with 4 they will be touching. Three will have their own space. We have our own line and have had for decades. One of the results of the breeding is that they get along with other dogs. We have no fighters; they know what they are supposed to do: hunt.
Anyway, that's been my solution since about 1995 or so.