Just a thought, I have fixed a seat belt buckle (ebay) to the seat frame , using existing screw hole. Reversed it so dog is clipped behind seat . Easy to use and safeSome time ago I saw self build van conversion with a rear bed with two dog cages under it. The owners showed collies, fly ball I think.
It did restrict the amount of storage under the bed but it meant the dogs were safe en route and could be let into the van without going out side.
I have the problem with securing our dog on route. Because our motorhome is a rear lounge there are only the front seat belts to fasten him to. He dosnt mind because he can sit on his mat between the seats but we have to step over him to get in the back.
I am thinking of putting a lashing eye on to the seat frame. He isn’t a very heavy dog.