hehe nice box sxtnitehawk, I haven't seen kitchen handles used on a pc box before.... not my cup of tea, but at least now I know what it looks like . Also to note, when nailing up the box with 12mm mdf (thats half an inch?) make sure you stay like 50mm (2 inches) away from the edge of the board, otherwise it will split. Also since you are using 12mm board, it will prove to be quite difficult not to skew the nails (coming out the side of the board), make sure the nailgun is exactly on a 90 degree angle, you've got plent of time so just take it slowly. The smaller the nails you use the easier it will be, but when trying to make the board flush with a mallet or something, the smaller nails will tend to bend and the board will come apart. 40 - 45 mm is a good length.
I don't know much about fiberglassing yet :P, I didn't think holding the corners with it would be that good though, does it even stick to mdf?
sxtnitehawk, what is a Drywall T square? and what do you use to fix the the carpet down? I would use contact adhesive stuff, spray it on the carpet and on the box.