Whenever I'm doing the straightening of these cardboard pockets I also:
-(sometitmes) sneak a flat piece of steel or aluminum around the edges at the top (pop-rivetted thru the cardboard)
-(always) put 90 degree angle strips (or just short brackets) of aluminum or steel at the bottom (that's where your arm-weight causes the pockets to sag to the point that the screw holes are "raised" a half-inch above their original location) and then provide new screw holes thru the metal
--- both of the above I've managed to hide under the leather or carpet (vinyl might be a little thin to completely hide it) by peeling it back first and then regluing with 3M upholstery glue.
-and I also pop-rivet on a double layer of tin or thin steel to the perpendicular brace (where the handle attaches) because I've found this keeps the thing from splitting at the single (double?) screw (bolt? --- can't remember) and pulling out at the handle area (I can't recall if my 74 had this or not but my 82 does). I paint it black when done.
Nobody ever notices any of these "improvements" and my pockets look almost new (no sag) and last a lot longer.
I've never tried the "steaming-method" --- sounds like a good idea... I've usually eased mine into shape by slowly adding the "bracing" (that Porsche should have put in there in the first place!)
On the other hand you can buy the expensive fiberglass units but I alway go for the cheap fix since it's more of a challenge (fun
) than just "buying" perfection.