this is the finalized version of the page you have seen earlier. You need to make a new upload to update files, you can find the comments of that now deleted page here: https://i.imgur.com/ZZEiQW4.jpg
I have tested some shaders before and ran into issues with custom shaders not working or in the case of this problem having geometry be visible while inside an inverted sphere with a skybox shader. So I guess you could build something similar with a panorama picture and skyboxshader if you place it outside the cameras far clip plane. The cubemap looks just as good as the panorama, you need to make sure it is reasonably large though so that moving your head left and right wont start distorting things.
Using a custom shader will not work on the Quest and by using the skybox/cubemap shader you run into issues with the Render Queue since you cannot manually overide it in VRChat. You were able to see the geometry through the box with the skybox you were in. This solution is very simple and likely futureproof as well.
after you doubleclicked your scene file check the hierarchy and drag the Trail position to the end of the left finger:https://i.imgur.com/XJkRfVg.pngThen click on the trail position again and set the Position coordinates as follows on the right side of the window in the inspector tab:X: 0 | Y: 0.0001960024 | Z: 1.399791e-05https://i.imgur.com/ZPt9oAU.jpg
I just downloaded and reimported the package again myself to test and see, its all working perfectly. Try making a new unity project from scratch, import the VRCSDK, cubed shaders and dynamic bones (if you have that). Then after you have done that import this package and look for the Mayu19k Scene file with the Unity logo ( https://i.imgur.com/5BJ9MlX.jpg ) and doubleclick it. Check if the Model in the new Scene (Hierachy -> Mayu19k -> Mayu19k) has a VRC_Avatar_Descriptor (see on the right in the inspector tab). If not add it by clicking "Add Component" and searching for VRC_Avatar_Descriptor. Then set the following values:https://i.imgur.com/93zjl9d.jpgBut as said, making a fresh project and following the instructions closely should be all you need. It may also be just Unity bugging out.
I have run a test and let some friends try it, it all went well for them. Try making a new unity project from scratch, import the VRCSDK, cubed shaders and dynamic bones (if you have that). Then after you have done this import this package and look for the Mayu Scene file with the Unity logo ( https://i.imgur.com/ewNUjJf.png ) and doubleclick it. These are the steps I have done to test the package.