This is my version.
I had a few of these corner braces left over from some prior project. They are 3 inches on each side. A package of 4 costs under $4 at Home Depot.
Now, I know what you're thinking. L-shaped pieces of metal are not so useful for cable management.
A hook is much better suited for this sort of thing. Modifying the braces is relatively easy with the right tools. Most things are...
Step 1 - Clamp the corner brace in a vise.
Step 2 - Use a pair of pliers and bend one leg of the brace 90 degrees.
Step 3 - There is no step 3. You are done modifying the brace at this point.. I said it was easy. Now repeat steps 1 and 2 for each of the braces you will be using.
Finally, attach the newly improved braces to your desk or table.
There are a couple options. You can use the top hole on the long side of the hook to screw the hooks in place. I ended up going with a less permanent option. This is mostly because I was attaching the hooks to the metal legs of my workbench and didn't want to drill holes. I used a strip of double-sided tape on the long side of the hook.