Then don't do two 15's then!
You want to highlight the quality and work of your shop, at least have the foresight to use speakers that will perform well in that space.
Show that you not only have the knowledge to physically build a custom setup, but you have the knowledge to put together a quality sound system.
I definitely agree. Unfortunately for him, it may come down to the demographic he's after. Is it the fool who errantly thinks bigger is better? Does he want to educate potential customers with a well planned quality build that out performs the raw & bigger & (not) better build? Judging by his choice of a Sound Qubed Hds300 15" sub and a Mmats 3000.1 amp, its the typical fool because that is not quality equipment.
If it were me, I'd build a 12" JL based system cleanly ported through the rear deck using a stealthy 6x9 grill, giving the impression of only a third deck speaker, while building sleek shaped sub enclosures that leave easy access to the spare. That shows people that they can have a badass build that looks like it belongs there while not losing any functionality. In my opinion, that speaks more positively about your shop because any shop can force feed two 15's into a trunk so I don't see the draw to make someone want to go to your place over another.
Of course, I don't the audio scene in Trinidad so your choice will come down to what helps you pay the bills. Either way, quality always speaks volumes.