float3 rayDirection0 = float3(0, 0, 1);
float3 rayDirection1 = float3(0, 0, -1);
float minDistance0 = 10000000.0;
float minDistance1 = 10000000.0;
float3 minDistanceNormal0;
float3 minDistanceNormal1;
bool didIntersect0 = false;
bool didIntersect1 = false;

for (int i = 0; i < _TriangleCount; i += 3)
{
    Triangle tri;
    tri.a = _MeshVertices[_MeshTriangleIndices[i]];
    tri.b = _MeshVertices[_MeshTriangleIndices[i + 1]];
    tri.c = _MeshVertices[_MeshTriangleIndices[i + 2]];
    
    float3 outNormal0;
    float distance0;
    if (IntersectsRayTriangle(rayOrigin, rayDirection0, tri, distance0, outNormal0))
    {
        if (distance0 < minDistance0)
        {
            minDistance0 = distance0;
            minDistanceNormal0 = outNormal0;
            didIntersect0 = true;
        }
    }
    
    float3 outNormal1;
    float distance1;
    if (IntersectsRayTriangle(rayOrigin, rayDirection1, tri, distance1, outNormal1))
    {
    	if (distance1 < minDistance1)
    	{
    		minDistance1 = distance1;
    		minDistanceNormal1 = outNormal1;
    		didIntersect1 = true;
    	}
    }
}

if ((didIntersect0 && dot(minDistanceNormal0, rayDirection0) > 0.0) && (didIntersect1 && dot(minDistanceNormal1, rayDirection1) > 0.0))
{
	_Voxels[voxelIndex].isSolid = 1.0; //or whatever
}