An exploded view of a similar one from ebay showing the parts.  There is a square chunk cut from the upper closeout plate. The top drip cap/retainer is shown at the bottom.

Side view of the chimney side =>

- click on any image to enlarge -

Reproducing a cover

End result (reusing old data plate)

Dimensions of the chimney from the side (lower access door removed)

and front

Close up view of the upper closeout plate and retainer (drip cap) detail

View with the bottom access plate removed

View looking up the chimney with the lower access plate removed.  On a new heater replacement, the exhaust pipe would not be present. Instead, the heater exhaust would direct to the opening in the lower access plate and would exit at the top of the chimney.

View of the back side of the lower access plate and the cutout for the exhaust.

One of the 2 Camlocks that holds the lower access plate installed

Most any sheet metal shop can fabricate one from novelty and perforated aluminum stock on a Pan Brake from these pictures and dimensions from your trailer opening.  Extrusions for the opening surround are available from Hoskin & Muir.

Style 2

View with the lower access plate removed

With the cover entirely removed showing the heater, which was inserted from the outside and the flanges sealed into the openings.  You can also see the big old heater exhaust - the reason for the larger external HW heater shrouds.

Front view of the lower access door - made from perforated aluminum formed into the shape of a pan on all four sides, except the top has a lip - see next photo.

Backside.  Almost any latch mechanism can be adapted to hold it in place.  The top has a lip shown here that tucks into the shroud opening.