I just made a 12x12 with expanded steel and it does not fit in the fire box. It's too big. I get the basket inside the firebox but can not close. It may be perfect if it's 10 x 10. Now I gotta look for other ways to make the basket as I don't have the tools to cut up expanded steel. I was doing it with a hand saw. Not going to do that again.
How tall are the sides? Where are you placing the basket such that the lid on the SFB doesn't close. I ask because mine is 12 x 12 x 6 and it fits just fine. I put 1" straps that sit just above the rails to hold the basket and it leaves room for the lid to close.