I loooove cast iron, and can't resist cooking with it and acquiring interesting pieces.
If you want to cover the whole grill - check out the cast iron griddles they use on paella burners.
http://www.thepaellacompany.co.uk/griddles.htmlOtherwise, just get any sized cast iron skillet or pot which suits - BBQ Pit boys on YouTube do something like this.
Something like a Sizzle Pan is ideal and relatively cheap. These are like the little pans you get tandoori served in some indian restaurants where the cast iron pan is pre-heated, and they bring it to your table sizzling. Something like this
http://www.amazon.co.uk/Sizzler-Steak-P ... B000Y9TVXOYou don't necessarily need cast iron either - you can use just a black iron griddle pan which is thick steel (not stainless)
http://www.amazon.co.uk/India-Bazaar-RE ... 00K3OWKD8/