With my girl, I say hold on...haha. She hasn't been restrained, but it wouldn't be hard to do. She moves between the passenger seat and the platform and/or a combo. If my wife or another passenger is along, then she lays on the platform facing forward or gets on the lap of the person in the passenger seat.
I went with a removable concept with a form-fit shape to fill in the areas between the seats, allow seat belt access, allow transfer case shifter access, and since it is form-fitted, it needed to hinge so I could swivel the seat. It is plywood, aluminum square tubing, and some 1x2 for the base and top support.
Also, since it is removable, I use ratchet straps to hold it in, mounted horizontally across the bottom. I put some heavy duty L brackets on the seat mount studs.
Also, I put an additional pad on top with a towel on the top of that. The carpet in the photo is padded and there is an additional pad under that.
Unfortunately my van is in the shop with the doghouse taken out, etc. awaiting a likely serious repair so I can't go take photos. That said, here are a few. I have the actual platform/console here at our house.