So feet were mentioned before. What hight feet should be under the sub, if any?
If I were to add 1" in any direction, how much would that effect the graph?
http://www.parts-express.com/re-audio-sex15-v2-4-15-cast-frame-dvc-subwoofer-driver-4-4-ohm--268-8075
If I spend more, do I get more?
In your diagram, are there 2 front boards? 1/2" or 3/4" wood?
Besides built cube and sub, what else is needed? Are the ports just open holes or do I need more?
My suggestion to add feet was based on a down-firing design, using the subs as end tables. If the subs are only 20" high I feel safe assuming that you're not going to use the subs as end tables, so I did the SketchUp mock up as front firing. If you do front firing, then feet aren't crucial.
Adding 1" to a dimension would give you ~0.15 - 0.2 cubic feet internal volume. The change in response would not be significant.
That RE Audio sub has a higher resonant frequency (32Hz if I recall correctly), and is more appropriate for car audio.
You might get more if you spend more, but there are different compromises to consider. The
Stereo Integrity HST18 is capable of output near 120dB, but it requires a box around 5ft³. The
HST12 has flat response down to 20Hz in a box that's only 1.25ft³, but it requires gobs of power to match the output of other 12" subs.
In my diagram, there are two 3/4" front baffles. The driver mounts to the inner baffle. The outer baffle serves to strengthen the box, and to make the driver flush mounted for a cleaner look.
In my Sketch-Up I made the port a 1" x 18.5" slot vent, somewhere around 36 inches long (I can't remember the exact length) to tune the box to match the resonant frequency of the Dayton 15 HO driver. As I mentioned earlier, you might prefer
this 3ft³ knock down box, adding your own vents. In that case, the vents would be two 3" PVC pipes with PVC elbows to extend the vents to the needed length of 27 5/8".