Plexiglas Skylights are created when a heated sheet of plexiglas is draped over a frame and air is pumped in from below.  When the shape is reached the air is stopped and the Plexiglas cools down and sets it’s shape.  My Aircraft door frames had a very irregular shape which required me to elevate the tooling. I first built a plywood base using 2 X 4s to support a base layer of plywood.  I then attached double plywood walls that supported the underside of the steel door frame. On the outer plywood wall I added a plywood flange which would provide a lip for the plexiglas to be clamped to. This flange also created a recess for the steel door frame to rest in and was flush with the face of the steel door frame.