Using CalculateFrustumCorners to raymarch in VR

CameraRaymarch.cs

static Matrix4x4 FrustumCornersMatrix(Camera cam, Camera.MonoOrStereoscopicEye eye)
{
    var camtr = cam.transform;
    cam.CalculateFrustumCorners(new Rect(0, 0, 1, 1), cam.farClipPlane, eye, frustumCorners);

    Matrix4x4 frustumMatrix = Matrix4x4.identity;
    frustumMatrix.SetRow(0, camtr.TransformVector(frustumCorners[0]));
    frustumMatrix.SetRow(1, camtr.TransformVector(frustumCorners[3]));
    frustumMatrix.SetRow(2, camtr.TransformVector(frustumCorners[1]));
    frustumMatrix.SetRow(3, camtr.TransformVector(frustumCorners[2]));
    return frustumMatrix;
}
 
 void UpdateCamera(Camera cam, Material m) {
     m.SetMatrix("_FrustumCorners",  FrustumCornersMatrix(cam, cam.stereoActiveEye));
     if (cam.stereoTargetEye != StereoTargetEyeMask.None && Application.isPlaying && UnityEngine.XR.XRSettings.enabled && UnityEngine.XR.XRDevice.isPresent) 
        m.SetMatrix("_FrustumCorners2", FrustumCornersMatrix(cam, Camera.MonoOrStereoscopicEye.Right));
 }

raymarch.hlsl

struct v2f
{
	float4 vertex : SV_POSITION;
	float4 interpolatedRay : TEXCOORD0; 
	UNITY_VERTEX_INPUT_INSTANCE_ID
	UNITY_VERTEX_OUTPUT_STEREO
};

uniform float4x4 _FrustumCorners;
#if UNITY_SINGLE_PASS_STEREO
uniform float4x4 _FrustumCorners2;
#endif

v2f vert(Input input) {
/* ... */
    int frustumIndex = input.vertex.x + 2 * (1 - input.vertex.y);
    #if UNITY_SINGLE_PASS_STEREO
      if (unity_StereoEyeIndex == 0)
        o.interpolatedRay = _FrustumCorners[frustumIndex];
      else
        o.interpolatedRay = _FrustumCorners2[frustumIndex];
    #else
      o.interpolatedRay = _FrustumCorners[frustumIndex];
    #endif
}

FragOut frag(v2f i) {
    UNITY_SETUP_INSTANCE_ID(i);
    float3 rayDir = normalize(i.interpolatedRay);
  	float3 rayOrigin = _WorldSpaceCameraPos + _ProjectionParams.y * rayDir;
    fixed4 color = doRaymarch(rayOrigin, rayDir);
    /* ... */
}

I'm not fully following implementation of these snippets... my raymarching implementation (not using uraymarch but similar I guess?) is multi-pass only, but using the code above on the camera/mat and setting the vert to the "o.interpolatedRay = _FrustumCorners[frustumIndex];" I'm getting garbage rendering for some reason... not sure why