Look on forBbodiesonly, forEbodiesonly (I suspect they would be similar) look at all manner of aluminum extruder, Place a wanted ad here, I suspect early As would be similar.
A machine shop could mill one in flat stock that you would need to slightly curve. A body shop with a metal stretcher wheel could do that. Or you might be able to screw it on and it might follow the curvature.
Either way there were only about 12,000 total 67 thru 69 dart convertibles made. So finding one will be a challenge
Good luck
