A possible alternative is the Mercury Marine covers used on the 194/230/250 motors installed in boats. They polish up like aluminum and are finned. I think they are embossed with "MERCURY MARINE", but that could be ground off or just left. The good thing is they are usually on junk motors that aren't worth rebuilding at the boatyards and don't cost much. The last set I got for free if I'd haul the whole motor away. Just call the local boat motor/engine rebuilders or boat repair shop and see if they might have a Mercury Marine straight six chevy and cross your fingers. The Merc Marine valve cover is the same story.
