So I took a broom stick and glued a piece of the same black carpet I used for the speaker board. This gives me a "curtain" I can put on top of the board while parked. Went to Home Depot and got 2 tie brackets for less than $1 each. Formed them into a "2" shape and painted them black. I can now park and put on the "curtain" to hide the speakers.
I think it came out very good and hides well. Here are some photos of the project.
The speaker board:

Speakers in the back seat:

You can see the brackets at the top of the ray of sunlight:

With the curtain up, all looks like one solid black carpeted area:
