Is there a way to get this component to extrude along negative z instead of positive z?   I tried a quick hack to  Matrix4x4.TRS() to use Vector3.back instead of forward, but the normals ended up getting inverted also, which kind of broke all the collisions.

Hi, sorry for the late response, The only way I can think of doing it is setting the PolygonCollider2DToMeshCollider.Offset.z to a negative value and still give it a positive value in "Thickness". Let me know if I understood your question or if I'm off.

In contrast to the default: