The intention was primarily to cover up the K frame & struts, not so much for aero, though that was arguably a side-effect.
Amblewagon made them along with the wheelbase conversion, though the Missile guys made theirs when they converted their RO/A990 hardtop.
It was an .050 aluminum sheet, hand-bent, fastened by sheet 1/4" metal screws at the rails & K.
Some of the guys removed theirs as time went on. At the time, weight was more of a consideration, and some of the cars got down to 2400 lbs.