Wow, I've got a project exactly like yours going right now! I also have a pair of the TC-12 drivers and am building 2.5 cf (gross, I figure 20% loss when factoring in driver and bracing volume) sealed boxes for each of them. I'm building them out of Birch plywood with MDF bracing. I'm using polyester fiberfill for stuffing, 2.5 lbs for each cabinet. I read an article (can't find the link, saw it on avsforum) that said polyester fiberfill is about the best stuff (if there is a 'best') to use for a sealed enclosure, and adding 1 - 1.5 lbs. per cubic foot of displacement makes the driver think it's in a roughly 25 - 30% larger enclosure. So, any fabric store should have the polyester fiberfill (picked mine up at a fabric store just a few hrs. ago), or Wal Mart if you can stomach that place. I'd use 2lbs in each box.
What are you using to power them? I've got a Buttkicker for each driver. Someone on the TC sounds forum just built a 2 cf sealed box for a TC-12 and was impressed with it's output, so I'm pretty stoked!
Edit: Found the link to the stuffing article:
http://www.diysubwoofers.org/talksho...ages/41415.htm
Also, I asked the same question you have over on the avs forum as well:
http://www.avsforum.com/avs-vb/showthread.php?t=839327