The .33µf 400V capacitor referenced in the original guitarnuts shielding plans is there for safety purposes only. It is there to (hopefully) block the high voltage in the event line voltages are applied through the guitar output, which can occur with certain types of tube amp failures. The idea is that the cap blocks the voltage before the voltage fries you.
The cap has no effect on hum reduction itself, and is not in the signal path, so it won't affect your guitar's tone. You can use any cap of .33µf, 400V rating, the material doesn't matter.
EDIT: Clarified at the suggestion of ChrisK who rightly points out that the cap blocks the voltage but doesn't actually destruct in the process. Thanks for the clarification, Chris, I was inaccurate.