Common computer desk has a slide out keyboard tray. I think it is not comfortable because we have to place our hands straight toward keyboard and our head should face the monitor at about 45 degrees.
Then I make this one out of junk. Let's get started.
Step 1: Swivel Wheel's Base
Step 2: Bending
Well, don't give up. You can see that I bended it in unperfect shape, just put both holes together. You can take it to a blacksmith or auto garage to bend it in good shape.
Step 3: Make It Firm
The middle hole is bolted to the swivel wheel and now I have four points to hold a piece of wood where the keyboard will lay.
I use any wood I can find and it is rough with the length of my keyboard and the swivel wheel side by side. For home desk you can look for a better piece of wood. Wider is good for your palm rest, but not too wide because we want to hide it under the desk aren't we? Find good piece of wood, but not too heavy.
Now drill four holes on the wood according to the adapter holes.
Step 4: Appropriate Bolts, Nuts and Screws
First, screw the swivel wheel to the bottom of the desk. Make sure the position of the tray before you screw, put some marks on where you should screw.
Then put the tray on the swivel wheel.