obi...
for the price of the sonance inwall subs, you need to look into the triad inwalls...they are built in to there own cabs already...and come in a variety of price points and sizes...
they will do you justice, and will outperform (albeit for the same or more cost) the sonance by a large margin...
this is their big boy...although it's 15" deep and meant for mounting in a pre-determined location...they also have a full line of 6 and four inch deep subs
[Link: triadspeakers.com]also, for what it worth, if he won't spring for the Sunfire (great sub) look into the Pinnacle BabyBoomer and BabyBoomer Plus...here...
[Link: pinnaclespeakers.com]GREAT bang for the buck, out if incredibly small cabinets...
good luck,
avdude