I built a box/kennel out of plywood in the pickup bed. It was in front of the hitch to the front of the pickup bed and stretched across the width of the bed.
The lid was split 70/30 with the 30% on the street side. The lid stuck up above the bed rail 3 inches. I put an RV roof vent near the center on the 70% lid for ventilation when stopped.
In the side of the box facing the hitch I cut a large eliptical hole and stapled wire mesh on the inside of the box to cover this hole. This provided the two medium sized dogs a view when the truck was parked(they slept in the box at night). Two cedar & foam dog beds plus water dish on the floor of the box.
The dogs were Keeshonds and with the type of fur they had, they hated to be inside. They would get too hot. The box provided them with plenty of air.
They travelled in the box while the 5th wheel was being towed down the road.
We never travelled in hotter than 75 degree temps with the dogs in the box. If the temp approached 75 we would bring the dogs into the back seat of the crew cab. They hated it but it was for their own good.
They travelled like this for about 9 years before they failed from old age.
All you had to do was say let's go camping and they would head for the truck.