Hi Cale,
You are correct in that 68 center A/C vent parts are not reproduced. The 68 center vent is different than the 69 center vent so the parts do not interchange.
If you are not worried about an OEM correct center vent cradle I may be able to help. I needed one for my 68 Chevelle so I engineered a functional center vent cradle for 68 Chevelle dashes that works with the Vintage Air A/C adapter. I used a 3D printer to make it. I print it in PETG plastic which is robust enough to handle temperature extremes, UV exposure and is impact resistant. Its properties are very similar to ABS plastic commonly used in automotive applications. I also have an extra center vent louver in good shape.
Message me if you are interested.
Respectfully,
Ted Smock