Question Hole interior / portal

Any tips on how to make these holes look more realistic? We cannot make an actual mesh that cuts into the wall due to some other limitations, so looking for a shader trick. I'm hoping it'd just look a little less like a flat black texture (which it is!) but almost like the start of a dark tunnel.



I found this but wasnt sure if it's the right thing. Seems a bit too advanced for what we need.
https://wiki.amplify.pt/index.php?title=Unity_Products:Fake_Interiors/Manual

 

I personally would use Stencil Buffers. Not too sure how you'd best implement that using URP/HDR/ShaderGraph or how useful it would be for your specific project, but for Built In here's a short article about it: https://www.ronja-tutorials.com/post/022-stencil-buffers/

 

I forgot to mention something important - we're on deferred renderer.