Don't use pop rivets.
Use elastic.
Make the armour slim fitting.
If the detonator pipe is black, paint it grey. Yes you can use humbrol. But spray would be neater. Or you can upgrade the detonator kit to a more accurate size and natively grey pipe.
Ideally the split rivets should be brass and painted white.
The AP helmet can be reworked into a hero, but technically it isn't a hero and will never look as good as one intended to be a hero. Talk to TM for the best hero helmet available.
The mp3 loop you're referring to is available to 501st members on the main forums once you have been approved.