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.

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Then place the cube against the bit with the hole.

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Jump on and next to the hole,

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And shoot the necessary portals to line up the laser.

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Proof of having completed it without using the cube:

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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!