I had shelves I made and never used, perfect fit for this set of rails. I recently got a fancy push a button to elevate a desk to insane heights. but it had the motor rail mechanism about 11 inches in. I tried one of those fancy swivel adjustable keyboard mounts, but since I had the above issue, I mounted the track 90° from recommended design, and it just bounces a lot when typing.got these rails (12"), absolutely perfect fit. mounted tray on rails, marked where the screws would go. drilled piloSee full review