Got it.
You know these large hatches taper so the bottom is bigger that the top. With that here you go.
Bottom, max dimension
20 1/2" x 15 1/2" except the tab makes it 16"
Depth, or thickness of hatch is 2"
The top surface out to the aluminum frame where it begins to slop down
19 1/4" x 15" This would be the top flat surface.
So If I were making a box it would be
20 3/4 to 21" x 16 1/4 to 16 1/5" x 2 inches deep
Hayden