I have a stealthbox with a 13w6 in my car, the fitment is tight and clean looking and sound is great (best of all i still have alot of trunk space). Personally i would go the stealthbox route and avoid building a box, it will be easier for you and the sound quality and look in my opinion will be much better.
2005 Mustang GT
Go:Whipple HO with Steeda 10 Rib, Hot Rod Cams, 1 Piece Driveshaft, Borla Catback, Aeroforce Gauges, Full Steeda suspension, MGW Shifter, Kooks LT Headers Catted X, TR6060,14' GT500 Brakes
Show: GT500 Conversion, Trucarbon Hood,Spoiler and many others, Side Scoops, Roush Vent Pods,Custom Valve Covers, 20" Saleen Extreme Rims, Kenwood Touchscreen Deck, Sequential Taillights.
HP: Enough to keep a smile on my face!