Help me with my subwoofer box

[BB68Vett]

I recently purchased an 8" subwoofer for my '68 convertible. I plan on building my own subwoofer box and will locate it in the rear compartment area behind the seats. I currently have two 5 1/4" ADS 300i speakers in a sealed enclosure. My plan is to build a large box to house all three speakers (the sub in the middle and two ADS speakers on each end of the box). I planned on partitioning the box internally so that each speaker would have its own chamber. I can build a box that will fit in the rear compartment and almost have the air volume necessary for the speakers (but I planned on using polyfill in the sub enclosure to increase effective volume). Now my questions: 1) Is it prudent to build one big box and partition internally or will the subwoofer affect the performance of the ADS speakers? 2) What direction should the subwoofer face? Forward in between the seats or should I face it upward into the convertible top and decklid? 3) If polyfill will effectively increase the volume of the sub enclosure, will it do the same thing for the 5 1/4" ADS speakers? Thanks for any replies.