IMHO, do make it all of aluminium and then fasten a multi-gun bracket at the back through to the joists. The law is satisfied and you get to keep your aluminium safe (if only as a high-spec dust cover).
I agree that it would be hard to break into, and that if someone really desperately want in they will get in - steel or no steel - but it is a matter of ticking the right legal requirements.
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