Maybe my friend Reepblue can help with that.

Yes, you're supposed to place a portal halfway above and below the grating (the other portal can be anywhere). Put the cube through the top half, bring it under the grating, and pull it through the bottom half. Unfortunately, I don't have the expertise to create a vacuum to make it easier.

So I am suppossed to grab the cube from above? Maybe you should create a Vacuum between the floor you walk on and the lasers below, that way the cube would get sucked into it and you could grab it from above. That is how the Pneumatic Diversity Vents work.