I use the bottom 16" cut from a 55 gallon plastic barrel and soak cook grates up to 22" in a water and Oxi-Clean powder.
I also toss any stainless steel and plated rotisserie parts like the Rib-O-Lator trays, EZ-Que cradles, Oct-O-Forks, baskets etc. in as well.
Usually an overnight soak is enough to remove most crud and the longer the soak the easier to clean. I hose them off in the tub. Look like new again!.
If $15-20 is too much to spring for a grate we need a new passion!