If it is super cheap, that top grate should be stainless steel. So, you might grab it for parts if it is cheap enough. Then, measure the hole & see if you can do what some suggested about cutting a circular hole & putting a lid upside-down in it's place.
If you are lazy, look for a place that sells (or uses) SS sheet metal & try to buy a scrap piece larger than the hole. Put it on the bottom & place some bricks on the edges to hold it in place and then just use it. Better yet, keep an eye out on craigslist & whatnot for someone throwing away something made from thin walled SS that looks to be large enough to cover the hole (maybe the back of a gasser that has a cabinet underneath).