NOTE: Apparently this isn’t meant to be done, as you can’t get the cube through the door. Ha Valve, been outsmarted!
First, grab the cube and walk to it.
Then place the cube against the bit with the hole.
Jump on and next to the hole,
And shoot the necessary portals to line up the laser.
Proof of having completed it without using the cube:
Don’t know why I did this. ALTHOUGH: done entirely in game (Steam overlay) and I definitely remember where my screenshots are (/userdata/[STEAM ID]/760/remote/620/screenshots). Hooray!