Creating a wall light

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~(A work in progress Feel free to improve)~
Introduction
In this tutorial I will show you how to make a wall light that is seen in the Portal test chambers.
Steps
Step 1
Create a room for the light to be in, in this tutorial I used a 384*384*256 room with 16 walls. Ensure the roof is touching the walls only by its edges.
Step 2
Texture the edges of the roof with the same texture you used for your roof.
Step 3
Shrink the roof by four on each end, so in this case, from 384*384 -> 376*376
Step 4
Create a 4*16 block with the invisible texture and put it right beside your newly shrunk roof. They should fit so there are no leaks.
Step 5
Copy the invisible blocks and re-texture them to have nodraw. Move them higher above the invisible blocks so you can easily preform step 6
Step 6
Texture the bottom of the nodraw 4*16 blocks to light_recessedcool?? Rotate them to be parallel with the block.
Step 7
Move the nodraw blocks to sit on top of the invisible blocks.
Step 8
Select all the top walls, and pull them up so they enclose the invisible blocks.
Step 9
Add an overlay with info_light_gradient, and change the vstart/end to .99 instead of 1.
Step 10
Resize to fit, and move inside of the hole.
Step 11
Repeat for every wall
Step 12
Compile your map, and ensure there are no leaks. If the light looks like the picture, then you have a leak.