Dunno if this helps, but just did something similar to the petrol-can hatches on my ex-mil 110. If you have the frames in place or can get them from an ex-MOD truck body modifying them to flip down like that would not be a big deal.
Mine are done in 1/8" ally plate with a stainless hinge and double latches - one top, one bottom, with the latter a lock rather than a thumb latch. it did take grinding off the old latch catch, but that was a triviality.
The actual construction if you don;t want to get too fancy was not at all hard and could easily be done in a home workshop with hand tools - if you get the door sheared by the company you're buying the metal from (I did not and cut it on power equiment - i have a pretty good shop).
ajr