I saw on another forum that someone put the cast iron grate from his Weber Q on top of the existing grate in order to get some good grill marks on the meat.
I can give you the link in a PM if you want it.
Don't know if this is of any help for what you are looking for, but thought I would mention it.
BGE grates would work sitting on the existing grate also. But I don't think you'll find any that fit. If you could, I think alot of people on here would already have them.
Are they locally available?
BBQ Outfitters in Southlake can get them. I think he makes 2 trips a week to his Egg distributor. I got my grate for my med in 2 days.
BTW, I think I worked a deal for them to sponsor me this year.:biggrin:
Maybe you could get a bigger square grate, cut ir round and weld on a ring on the outside?
Good idea but, welding to cast iron is a tricky thing.