I cut the out and insides the same so all I do is bend sheet metal to fit the curves and then weld it all up. One piece of sheet bent by hand to fit. Tack it in several places, The I take a flap wheel on a angle grinder and shape the sheet to the door.
The lock mech welded in the unlock position. Its on the bottom of the latch.
Tack on the back side of where the latch goes if not this part will slip and the latch wont work.
The latch handle. I used flat bar stock on this one. Drill a hole so you can plug weld it to the latch. On my neighbors taco doors I used 8mm Craftsman wrenches. They worked out great. slid the open end on the latch and welded it and you open the door with the boxed end.
I did the access holes because something is gonna screw up and you will need to get back in. Trust me.