I see ,I wouldn't use the one from amazon, it is only rated to 110lbs and was just an example of the type of shelf. I would look for one rated for more weight. This one might work and is rated for 350lbs
Adsu2536Bk 1U 25-36 Adjustable Telescoping Solid Shelf
Need an adjustable 1u shelf? Hammond carries a wide variety of depths and weight capacities. Check out the ADSU2536BK.www.rackmountsolutions.net
Mine does not not slide out, i just leave 5U-6U empty above the enclosure for access to the drives
hope this helps
I found one that is sliding an rated for the weight but it is too expensive at 359$ they make the deal not as good