If you buy the stock grates to weld legs on, you'll only be able to raise it a few inches since the upper cookbox is octagon shaped, therefore not as deep the higher you go up. Do you have the proper welding equipment to weld stainless steel?
I too am looking for a upper shelf. I'll probably just use carriage bolts for the feet since I don't have a TIG with gas to weld stainless for the legs. The largest that will fit is 16"-18"D x 24"-27"W at about 5"-7" above the stock grates. I'll probably go with some ~10"-12" deep so I can still have access the bottom grates.
Amazon has several grates with feet, though many of them don't list the size