You get what you pay for, simple as that. All those fancy black anodized cases are complete failures when it comes to RFI and EMI rejection. Anodized aluminum does not conduct electricity, so the box pieces are not electrically connected to each other in spite of being screwed together. You might as well have no box if you're going to use one of those.
However, if you want something that is rated and tested for RFI and EMI rejection, then these guys have what you need: Par-Metal
I've bought two of their Series 14 boxes, and I have on order a custom Series 20. They are reasonably priced for the quality that you get.
you are partly correct.
The seems must be in electrical contact.
Screws are an effective method but they leaves gaps in the seem joints.
The effectiveness of the enclosure is inversely related to the length of the longest seem gap.
If your parmetal enclosure manages to solve the seem gap (without electrical contact) then it is indeed a very good enclosure. But I doubt it.
Sorry this thread seems to have died. Here's another one in case anyone is still wondering, as I am. I haven't used them so can't vouch for them, but they look good and are offered at good prices and reasonable shipping.
Still wondering about other options, particularly metal with some venting.