Try to poke 1/4 inch flat screwdrive into the beams where the gap in the deckboards was. If it doesn't sink in more than 1/4 inch you are good. I second the "wrap the tops" suggestion.

If you can stand the look, using machine screws that go all the way through the rail would be a solid addition. With a counter sunk flat head on the exposed side and a nut and washer on the inside it would then almost not need clamps.

You might take a look at the variety of Bath and Kitchen exhaust vent covers. There are a number of styles and colors and they shouldn't affect sound from the speaker.

It has a current going through the liquid and into the metal object, that's why they call it "galvanized". The electricity bonds the zinc to the steel better than just dipping it.

Some items are just dipped in liquid zinc "hot dipped". This leaves more zinc on the item and tends to be more expensive.