You may need original parts
I am a long-term subscriber to Auto Restorer and am working on a 1963 Jaguar XKE. The car has round brake and clutch fluid reservoirs about 2.5” in diameter, 4” high, with a cap on top and spout on the bottom to hook up with the brake and clutch piping. New reproductions are terrible. These reservoirs cannot have been unique to Jags so other manufacturers must have used them—either plastic or metal. Perhaps a reader can steer me to a crossover part.
A lot of people assume that most of the parts on their collectible cars had to have been used on other model vehicles as well and that’s not true. While there are obvious efficiencies realized by using identical parts across a broad range of vehicles, manufacturers haven’t always done so. I could not find a cross-reference for the brake and clutch fluid reservoirs for your 1963 E-type. Even if there was one, however, how would that help you? Even if the identical reservoirs were used on another car of the era you’d still be in the same position. They would not be available new from another manufacturer and the available reproduction reservoirs you don’t like would almost certainly be the same reproductions offered for the other car as well. If you don’t like the repro units your best alternative may be to use originals instead.