Suggestions for heating box with canopy? really Need help!

1990 Dodge Ram 150 PowerRam 4x4 with 360. Moving across Canada in January and could hit weather -40 -50, have two dogs and need them to ride in the back of the truck inside the cap. (Children in the truck up front). I'm going to insulate the cap with styrofoam and am trying to figure out some way to route heat back to them. I've seen the cheap 12 volt heaters but they seem they might not do the job. Any suggestions? One dog is a Husky, and she's always fine with hay, but I'm concerned about the other one whos a lab.

any help at all is greatly appreciated.

I am not a big fan of using an electric heater for such a long trip. They are intended for short term use, until the coolant heats up. IMO they may eventually cook the alternator

You can get a stand-alone heater using engine coolant and having a 12 volt circulation fan.
These are intended for the rear of full size passenger vans. They are designed to bolt to the floor and use regular 5/8 inch rubber heater hose running hot engine coolant from the engine, just like any regular heater core. Also they have a 12 volt fan.

If you cannot fine one in the aftermarket, try wrecking yards, Ebay, etc to find a used one.

Alternatively, you could scavange the heater core, blower motor and duct work from a scrap car. If you find the right one, with a bit of creativity you could adapt it to work back there.