Cool, looking forward to pics.
If I did mine again I'd probably do as VC said earlier and find an engineering shop to fold the sheet twice so as to end up with sides and back done at least but yours sounds different.
I did look at Fletcher Steel initially to find their largest 4-sided square sections with 6mm wall thickness but from memory they only went out as big as 300-350mm wide. A couple of those welded together would be a cheap option but probably too small for most.
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