I already had one Pack Rack for which I didn't get the top cap. I also got another Pack Rack recently and that will go on my new one. I love the Pack Racks as they are mobile which means I can have what I need at hand without taking a trip across the shop.
This "Top Cap" for my Pack Rack was decidedly smaller than expected = it is 23" wide x 8" deep, where the overall top of the Rack is 23 x 12 = it left a big hole in my overall storage plan, so I made things work with a larger 1/2" plywood shelf insert, allowed for open areas at each side, now my overall top cap is 23" x 12"+. Needed a dollar $ worth of longer 1/4" carriage bolts/nuts to secure the deal together. The 8" side pcs are bolted through to original Rack frame, the 23" angles are secured to the plywood shelf.
Note that this only includes the four side rails. There is no bottom to it. You have to make that yourself. Not a big deal. The side rails should extend to the full size of the top. All-in-all, I don't think you get value for the price of this accessory.