You could consider using brush strips. We used to use them as 'seals' on some machine doors. They're not airtight, but hugely reduce the draught and are light weight so would not affect the door performance. We got them from Kleeneze
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. They come in different bristle lengths up to 70mm or so and strip lengths up to a couple of m. I put one on the front door of the old house and it made a big difference according to management.
Richard