Slicing a sphere mesh and applying reflection probe data in LWRP

Hi all, I'm trying to work some custom shaders into a pre-existing LWRP project, but have literally 0 experience with shaders. We're trying to mimic a convex vehicle mirror (that updates constantly), and currently have a reflection probe set up to provide the actual reflection, which is then displayed on a secondary Sphere mesh using a custom shader from here:

After the reflection is applied to the Sphere, I want to cut out the parts of it that aren't needed. I've looked into a few different methods to do that and the closest option I've found is using a separate Quad GameObject as a cutting plane using either this shader/script or this shader:

Problem is, I have absolutely no experience with shaders in general, much less when using LWRP - I can get these custom shaders to work in their own projects, but don't know how to tweak/merge them together or to get them to work with LWRP. Can anyone help me out? This shows how the sphere mesh should look (but pulling reflection probe data instead of from its own material), and then a cutting plane or something would be used to cut out the remainder of the sphere's mesh from view.