I posted over at the UMTR, but I'll reply here so others can see.
I cut mine up last winter after collecting pieces of material to do it. I made the reinforcement plate that welds to the bell. I tacked it to the bell and warmed it up and bent it and welded it solid. I then made the outer cover plate. A friend rolled a piece of material for the filler. I took the bell to work and used our big horizontal mill to mill the opening. I then ground the filler to fit perfectly. I hand drilled the holes and tapped them since they would follow the radius of the bell and not exactly perpendicular to the opening.
It is all painted red and looks respectable from under the car. I showed pictures to Chris Hyatt and he was impressed. I have not had it re-certified since I'm not concerned with NHRA inspectors poking their head under the car.
Not quite as fancy as Browell, but ....