VR Mirror with Unity XR - Does anyone know how to?

For my project I need a perfect mirror in in VR using Unity's XR system in URP. The problem is it uses a single camera and when following mirror/portal tutorials that use cameras to project onto render textures, the left and right eye don't match.

I've seen this shader: https://assetstore.unity.com/packages/vfx/shaders/urp-lwrp-mirror-shaders-135215 that implements it (I think) but its 22 bucks and I'd like to know how to make this myself.