Turdboman - again welcome aboard !
When you ask a generic question that could bring a fishbowl
full of replies, scattered in all directions.....
you would find it more helpful if you put more information
into what you are working with...... most "add-on" decks are 1/2" plywood.
Which would require additional supports from underneath.
If you are making an "all metal" deck, that is a fish from a different pond.
Hatch covers are in a family all by itself . . . I think each person tailors their
hatch covers to fit their particular applications, and most look and work quite well.
photos or drawings of your concerns will most definately get you more accurate responses.