Laying it out
The first thing to determine is how much of the cooker you want covered with insulation, and how much will be covered by the door. I used a door width of 22-1/2", which gave enough room to easily operate the cooker door latches, and open the doors completely.
Using a framing square, mark a line on the drum parallel with the axis, 2" away from the door hinge pins. This line MUST be square with the drum, or the jacket won't fit right. If in doubt, use the vertical seam on the drum as a guide. Measure 22-1/2" around the drum, and mark a second line. The sheet metal angles will attach to the drum here, and serve to cover the edges of the insulation and as a mounting surface for the door hinges and latches. Visualize the angles in place here, and make sure you have enough room to operate the cooker doors.
Here's another view:
You'll need to refer back to this drawing when we start talking about mitering and fitting the handles.
Fitting the angles
Measure between the drum top bead and bottom flange; deduct 1 inch. This will allow 1/2" between the ends of the insulation and ends of the drum for the caps. Cut the 4 angles to this length at a 45º angle, making sure the flange of the angle is the short side (the outside). Hold one of the angles against the drum along one of the lines marked previously, being sure to leave the 1/2" gap top and bottom. Transfer the outline of the drum's ridges to the angle. These areas will be cut out, so the angle fits flush against the drum.
Clamp a short piece of angle to the drum top and bottom at the line, then clamp the mitered angle to these. Tack weld the angle in place. TACK it, because you'll need to adjust it a bit, so that it's perpendicular to the drum. After it's adjusted, make two 45º gussets out of scrap, and tack them between the angle and drum ridges, on the insulation side. Make sure everything's straight and square, then add some more tacks. Weld it out later. Repeat with the other side. Remember, the leg of these angles goes AWAY from the door.
While you're messing with the angles, go ahead and do the other two. Cut the notches to clear the drum ribs, and clamp each angle to the ones already welded on, with a 1/8 inch spacer between. The legs of the two door angles face toward each other. Make sure the ends of the angles are even, and the gap is even between them.