baby seats don't require the support of an actually seat, thus, you can make a frame to directly tie the seats to the floor.
Good point, which reminded me what I did when my twins were born. We had a Mazda GLC, as in a TINY hatch back. The car seats would NOT fit in a rear facing position in the back seat. Plus it would have taken a contortionist to maneuver a baby into the back seat of a two seat car, and into a car seat. So I folded down the back seat, made a frame to hold the car seats and bolted it into the back. Access to the car seats was through the rear hatch. This worked for a few months until we could afford to change to a small four door car.
Don