If send myself through the force field all portals close. But I need to place the second cube to hit it with a rocket. The funnel is in the way.

youre missing something, think about what would allow you to move the portals from where you are

So I can use the first cube as a step, and climb into the funnel. Now I have reached the second cube. I cannot get the second cube as I cannot close the portal so the second cube would be destroyed.