My 1972 250 still had the original passenger side underdash cover when I bought the car in 1988. I salvaged the drivers side from a 250C
parts car that I owned at the time. I installed two 6x9 Sony coaxial speakers in these panels which sound much better than the
tinny 3" Beckers that were under the dash originally. I don't think there's enough room behind the kick panels to mount decent speakers unless you want to cut open some inner fender metal.

I think you can use regular fiberboard for the flat areas under the dash and screw or pop-rivet some thin sheet-metal to the edge. This can be bent down and screwed to the firewall. This is what I did for the missing cover on my 1960 220S. I used fiber carpet padding from Home Depot for soundproofing.
As many of the bendable cover retainer tabs were weak or broken on both cars, I chose to screw the outer edges of the fiberboard to the underside of the dash.

Happy Motoring,
Mark

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DrDKW