Step 1: Buy hardware
2 x 1/2" thick, x 6" eye bolt w/nut zinc plated
2 x 1/2" nuts zinc plated
2 x Carabiners (locking ones are probably overkill, but get good quality ones that latch into a hook system when closed)
4 x 1/2" lock washers zinc plated
4 x 1/2" cut washers zinc plated
(image shows disassembled parts on left and assembled version on right. The washers will go on either side of the sheetmetal of the car.
Step 2: Make the holes
With the upper seat cushion still in place, position the eyebolts so that they are flush against the bottom of the cushion and mark the area that the base touches.
Once the positions are marked, use a punch or drill to make a pair of holes about 1/2" in diameter. While this shouldn't be a problem in most cars, you should check to make sure there are no fuel lines, brake lines, or electrical wires located on the other side of the sheetmetal where you intend to place the hole.
Step 3: Secure the bolts
Once you have the depth set, climb under the car to place the remaining nuts and washers onto the opposite end of the eyebolt. Be sure to put the cut washer on first, followed by the lock washer and finally the nut.