Rusty sez:
INHO?  ;) "Barndoor"  is easier to say that "Large singular top hinging rear
hatch that intergrated what was to become the individualized rear cargo
hatch and the engine deck lid and spans in height from  just above the
bumper line to just below the rain gutter and was produced up to 3/55."
To old fossils like me we used to hear sayings similar to "it is as big as a
barn door." People used to be aware of what a barn was and the general size
of a door for such a structure. Folks also used to be less uptight (anal)
about figurative descriptions.  Folks also used to know what Shinola was. 
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hey, i got my tin of Shinola,
         but i still think it's a silly name
it's not too hard to say 'big engine hatch' (and more accurate than your
description,  which by the time i got that out i'd have forgotton why i said
it), 
but that doesn't have the same ring...

the idea still simmers on the back burner, waiting for inspiration  and
flights of fancy

clara