Yes, I actually DID have a close look before I replied with my suggestion. I was a Ford body shop mgr and then a Parts Mgr, in my 'previous life', so I am speaking from a little experience, anyway, lol. My idea would entail grabbing one that is a bit too large, and then snip -or grind off, the 'leading edge' on the side opposite the spring. That way the clip fits with enough 'offset', as it approaches the door's edge there.
If I had the time I would try and find you an Au-Ve-Co interchange number, as that company supplied most of those 'funky' clips back in my era. 'Clips and Fasteners' site is also a good one for that odd-duck item.
If you have an 'old-timey' body shop nearby, you might find a gray-haired and weathered looking sympathetic body man or manager who wouldn't mind checking through their 'misc clip drawers'. ....Ideas anyway ?? ......In a production shop time is everything, so what may seem like a 'hack' to one guy, saves the day to the next guy. LOL ...so we all kept boxes of clips, nothing usually got tossed.
I needed a teeny one for my '66 Fairlane last summer and found many sizes stocked at my local O'Reillys, but ultimately I found it on Amazon, go figger. When I putzed around online yesterday, NAPA came up with some too..... Thus, why I mentioned those venues.
I'm sure your 3D printer idea is a fine one too. I'd just coat the clip with some (dielectric or other) grease to avoid any possible future corrosion -if stainless isn't available,...... but that is me, lol.
