I haven't seen a 3D model but there are a few companies that make these adapters in aluminum, which I think is the right material for this application. Even with modern filament like PPA-CF I'd be worried about longevity due to close (with a phenolic spaccer in between) contact with the head, vibrations, etc.
Adapt is a bit cheaper than Islandworks
https://adaptmotorsport.com/collections/current-products/products/porche-964-intake-manifold-adapter
I only the GT3 intake version but I'm pretty sure they have one for the 964 as well since the owner Jonas has a 2.7 or 3L engine with 964 intake.
https://islandworks.se/products/billet-intake-runners-for-porsche-g-model-to-gt3