As promised some pics of the build of the cabinet.
If you don't mind, I'll talk metric. I'm dutch...
Started with 18mm (3/4") birch plywood.
Braced the cabinet in the center. Baffle to back, side to side. Made the braces a little bit larger then required. About 1mm (1/25 of an inch) So there's some tension on the panels. Should help reducing virbrations.
I thought 110mm(4") PVC drainage pipe would be a smart choice for the ports. With an extra supportpanel on the inside glued to the baffle. Bad idea. Lots of work, result isn't great. Standard 4" tube ports are available for less then $2 each. Looks better, can easily be adjust and gives the cabinet better looks.
I used 2 Celestion Q12 2550 8ohm 12" 300W BASS Speakers. I send Celestion an email asking for some support. Worked out great, thumbs up for their support. Fast and answered my questions.
They suggested 4 ports, cylindrical, 4"x4" to start with.
Volume of the cab is appr. 140liters (4.94cubic feet if I'm correct)
Finished it with Penn Elcom dishes, handles, corners etc. Speakergrill, speakercarpet etc. All it needs now is a logo...
The sound: great if volume is turned up. No sparklin high's. A little dampened. As I'm no native english speaker, don' no the exact words to descripe. And if I would be, it still would be hard

Low end sounds great, not a big fat sound like a 15" would do. Just as I wanted.
I'm pleased with the result.
Pictures by jojobeerten - Photobucket