Proper cubic foot box for MM6x9

mango3073

hello forum, I'm ordering a pair of MM6x9 for my golf cart. It will be a custom install with fiberglass boxes above ear level near the roof facing me (the driver) on both ends for true stereo imaging. I've read the pdf manual and it does not have a recommended box volume. I will high pass the speakers at a level so they don't play the extreme low bass (thinking around 50hz, but will play it by ear). so hopefully they will last for years. What box volume should I have the boxes made at? it will be powered by the polk 2 channel amp. Thank you guys and gals.