THATpurpleKUSH 5,000+ posts
Fuckyou
According to bass box pro a pair of those 6.5s only need about .38-.4cf tuned around 40hz to model pretty flat. You'll have decent extension into the mid 30s, don't expect anything below about 35hz. The higher you go above .4cf the more under damped the design will be and the more peaky the response will be. So smaller is better in this instance which is good because of the limited space. The hard part will be adapting a port. I still think a passive radiator would work best.