Nice idea, certainly better than my computer speakers under the seats! More recently I have used one of these sat on top of the dash which also handily does the hands-free for the phone:
https://www.amazon.co.uk/Anker-Sound...73&sr=8-3&th=1
That single speaker came out of the chromed grille that I fitted to the rear parcel shelf. There is a knockout in the body which is still in place - all I wanted was something smart to hold the (warped) plywood backing of the parcel shelf in place, so didn't drill the knockout to fit the speaker itself.