SpiralBlur-SceneTexture not to blur objects before the plane with SpiralBlur

Very simple translucent shader using SpiralBlur-SceneTexture:

And very simple setup with two opaque cubes and a plane with spiral blur shader. The problem is it’s blurring not just the objects behind the plane but objects before it too. Any ideas for workarounds?

could use a scene capture 2d. with it’s own post process material.