In progress. My main thread is at diyaudio.com (
http://www.diyaudio.com/forums/showthread.php?s=&postid=1308576#post1308576 )but I figured I'd post here too to draw from and hopefully contribute to the wealth of experience. Basically a dual driver ripole with the cones facing each other, . a.k.a. BMC (Blow Motion Concept)
Pyle PPA15
* Power handling: 250 watts RMS/800 watts max
* Voice coil diameter: 63.5mm
* Impedance: 8 ohms
* Frequency response: 27-4,000 Hz
* Magnet weight: 70 oz.
* Fs: 26.7 Hz
* SPL: 90.2 dB 1W/1m
* Vas: 0.321679377 cu meters
* Qms: 2.69
* Qes: .89
* Qts: .67
* Xmax: 6 mm
* Sd (approximate value for a 15" woofer with pleated surround) : 0.0856 m2
* Dimensions:
Overall Diameter: 0.382524 meters
Cutout Diameter: 0.352044 meters
Mounting Depth: 0.152908 meters
Here's a sketchup rendering

Critiques, . .suggestions etc .. welcome . Should I expect this device to be utterly inefficient?
If I need to add a U-extension at the rear later on, I plan to implement it via a sliding sleeve.