Pimax 5k/8k interface for narrow faces

This interface fits between the face cushion and the Pimax 5k+ or 8k to give a better fit for those with a narrow face and also reduce visual distortions (fishbowl look). Results are heavily dependent on face shape. *Also check out the [comfort mod](https://www.thingiverse.com/thing:4013583), which supersedes this but requires a custom foam gasket.*

Distortions were pretty bad for me on the stock Pimax 5k+, but I noticed that moving the bottom of the HMD out a few mm reduced them a lot. This