Use an Aerial Faith Plate to launch a turret
This, Final Transmission, and Drop Box can be gained in Chapter 2 - Advanced Aerial Faith Plate Test Chamber 6. Read ahead a bit, so you don't make any mistakes or miss them.
When you get to this chamber, play through it until you come to the switch on the other side and make sure both your portals are still on the inclined slabs. Now press it once and the tube back where you started will shoot out loads of junk. Wait for it all to settle around the chamber, near you or down below behind you and then look for the turret.
Once you've found it, and if you can from your current position, pick it up and make 2 portals so you head back to the start. Head over to the launch pad with the turret in hand and drop it on it.