I need a garage door for a space of 273w x 206h cm, and am currently thinking of a cheap DIY option to save some cash.
Would two sheets of marine ply, painted, each fitted to a 2″ x 4″ frame to prevent warping work out ok? Or would it be a total bodge?
(I know the dimensions don’t quite add up but I have some 8″ x 3″ to use as uprights to fix to the concrete/hinge and plan to use a 2″ x 6″ as an overlap where they meet.)

It will work but will only look as good as the effort you put in.

Most of the houses by me were initially built with wooden garage doors which have perished over the years. A neighbour of mine repaired the bottoms of both of his doors with 1″ ply and to make the repair look reasonable he used a router to put in grooves to match the original sections of wood.

Now painted it looks ok.

is marine ply that much cheaper than a quality T&G which will look infinately better??

my parents garage door are made of a wooden frame with 14 or 16mm T & G nailed on.

its 2/3rds 1/3rd barn doors.

That’s similar to my DIY barn doors

Hardwood ply with left over decking boards screwed/glued to it:

And… the ‘workshop’ door was similar. Two sheets of ply sandwiching a 2×4 frame. Very stiff construction.