Interesting thread here. The vent is always the killer when you are looking at small vented enclosures. They almost always require huge venting (which takes up big space) removing the point of the small enclosure.
Passive radiator may be an option but good proper ones up the price and affect the enclosure dimensions significantly.
HOWEVER
The best compromise I can come up with here is the CSS SDX-10 in 2.0ft^3 with 2 of the CSS APR10 passive radiators with 160 grams of added mass each.
This yields an extremely flat response (+/- 1db from 29.4hz to 70.7hz [lower & upper -1db points]). It also has pretty good extension too with a -3db point of 27.5hz (at 106.9db with 300 watts rms) and a -6db point at 25.2hz. The upper -3db point is 81hz.
This assumes the high pass filter in the amplifier and a 4th order low pass at 80hz. There is a slight bump in xmax nearing 20hz that exceeds by about 2.5 mm or so but this should not be too much concern for the driver at hand and power should be backing off some at that point because of the infrasonic filter.
I don't think you could ask much more from such a small enclosure.
Here is all you need for just $349.00
If this enclosure is still to large, you can drop to 1.5ft^3 and up the mass to 180 grams each. This option will raise the f3 to 30hz however so it hurts extension some.
