you can put screws under the end of the panel , it is hidden by the inner 1/4 panels , it may still buldge in the middle though
I ended up with 1/4 inch thick carpet pad under the panel, anything thicker won't let the panel lay flat, and won't let the panel slip under the inside trim for back glass.
I mocked it up, and yes, the ends of the panel are coverd by the inner qtr panels, and the qtr panel screws hold everything together. Also, the top of the rear seat back is above the flat panel, and the panel is held tight in place with the seat back in place, so 73chally and acudanut are totally correct.
Not sure if 74 is the same as other years, but there are no clips, the notches in the flat panel are to allow the screws to pass thru that hold down the inner qtr panels, and the very aft end of these qtr panels slip under the trim for inside of rear glass. The glass trim even has a step formed into it giving extra room for the inner qtr panel to slide into.
I found my old package try, and not in the dumpster!!. It is mopar oem, no metal clips or anything just the panel with felt glued to the backside, and has the notches which the aftermarket copied.