A subpoena is a compulsory order to produce all documents related to a matter, whether the government knows they exist or not. So yes, if you believe in subpoena power, you agree that you should have to produce that box.
The Fifth Amendment, as currently interpreted, doesn't provide the protection you describe. It protects against giving self-incriminating testimony and essentially the question is whether the password is "testimony." Keep in mind that the government is not asking him to disclose it, just to type it into the computer. Here's a good discussion: http://volokh.com/posts/chain_1197670606.shtml (There seems to be a CSS problem with that page, but the text is fine.)
You have the coordinates of the dead body, or they can dig and scan each square foot of the continent. This is not a Fifth Amendment case. They don't want or care about the coordinates itself. They can't use the information you provide (the coordinates of the dead body) to incriminate you, no matter what the coordinates of the dead body is.
> US v Hubbell makes clear that the government cannot compel production of documents (except under immunity to prosecution) which it does not know, ahead of time, to exist.
Dunno if that's true (as IANAL), just quoting.