Bed extender for 2014 Tacoma

I just bought a bed extender for my toyota tacoma. My extender comes in three parts. The videos all show two parts. Any recommendation on how I stop the center bars from sliding when I am installing this monster.

 

I believe you have to mark the center piece like with tape at each end and loosely tighten it together. Then throw it on your bed and adjust it to fit then tighten down when you have the correct fit. The tape is so you can evenly extend each end piece. At least that's how I did mine.