Cutting a hole in the camera

Hi!

Short version:

I have 3 groups of objects. I want to cut a hole in camera 1 and render camera 2 be$$anonymous$$nd it, using the red objects as clipping objects. All the objects are in the same scene. I've tried using shader graphs applied to the camera.

Long version:

So the above 4 part image is basically the effect I am trying to create. So I have a group of objects being rendered by camera 1 (picture 1), a second group of objects, in the same place in the same scene, being rendered by camera 2 (picture 2), with its own skybox (solid colours for an easy example). I've already done t$$anonymous$$s using culling layers. Then I have a t$$anonymous$$rd group of objects, the culling objects (picture 3), shown by the red sphere, with the group 1 objects be$$anonymous$$nd it for context, w$$anonymous$$ch could be black or any other colour, t$$anonymous$$s doesn't matter, as long as it's an object defining where to clip. Picture 4 is the end effect I am trying to ac$$anonymous$$eve. What I've done so far is having camera 1 rendering on top of camera 2 using the render priority. Now I'm trying to make a shader on camera 1 to somehow test all of the pixels, and if they match the exact color of a culling object, then render not$$anonymous$$ng for that pixel (including the skybox, w$$anonymous$$ch has been my toughest problem). Using any other way of determining where to cut pixels using meshes would be great! I don't know if having a color restriction like t$$anonymous$$s will be a pain to deal with later! Either way, then camera 2 would render normally.

If possible, I would like to do t$$anonymous$$s using the shader graph as appose to manually programming a shader. I would say I'm a pretty experienced coder, but not with programming shaders. I'm also using the Universal RP. As well, it would be great if I could have some sort of black border surrounding the holes or some kind of polish to it like that.

I've been doing a lot of research on t$$anonymous$$s, and can make normal shaders just fine, but unfortunately there is not a lot of information out there on applying shader graph shaders to the camera itself. All the images here were made with Photoshop, although I did make a shader to get the red sphere to render as all 1 colour and ignore lighting information.

If it does matter, I would then like to render UI elements on top of all t$$anonymous$$s.

Help solving any part of t$$anonymous$$s mystery would be very much appreciated! Thank you!