Article III of the constitution is amazingly brief. It doesn't even explicitly create the supreme court's power to determine constitutionality.

> It's really unfortunate that secret courts aren't unconstitutional.

The Foreign Intelligence Surveillance Court and the Foreign Intelligence Surveillance Court of Review aren't secret courts.

Their proceedings may be invariably be secret because of their subject matter, but the proceedings of regular federal courts may be, too. The difference is that the FISC and FISCR deal exclusively with issues that are handled in secret, but there is no principle that would prohibit them as "secret courts" that wouldn't also prohibit all sealed proceedings in the other federal courts.

the US can always pass a new amendment. (I mean, if you can persuade the legislature.)