Have a good look underneath the car again. Often around the subframe/whisbone area there is potentially a dual layer/box contruction for strength.
One you have worked out the exact layout then tackle the inside again. Have a tap around with a hammer to see if you can locate any hollow areas on the floor pan beneath the bitument floor liner pads.
You may find however that the captive nut is not accessible as it is welded onto on the the floor double skin outer construction layer.