Transformer covers; steel or aly?

[awkwardbydesign]

Is this the right place for this basic question? I am starting a #26 DHT preamp build, similar to Rui Lourenco's, but can't find the answer, and I have looked!
Will it make much difference if the transformer covers are steel or aly? The mains is a Hammond 370X, and the OPs are Lundahl 2745s. The covers will probably be quite close fitting.

 

[JonSnell Electronic]

The covers are purely aesthetics and have no function otherwise.

 

[hego]

Mild iron. Low carbon.

 

[Nikolas Ojala]

They should be some magnetic steel. That is the only sure way to close magnetic fields inside the covers. Ferromagnetism is essential.

 

[awkwardbydesign]

Thanks for the replies. I actually like the look of the steel ones I have seen, so I will probably go for them.