I just got the 12.5 cast iron Traeger one. Place on top of the current grates. I did a bone in ribeye at 225 for an hour then kicked it up to 500, once i pulled the steak off at 225 i placed the 12.5 Cast grate on in the middle of the grates in there and let it heat up with the T to 500 then through the steak back on 5 min sear each side did the trick. Here i s the result.
Now if you take the porc grates out you can place the 12.5 cast grate front to back the long ways and it firs perfectly, Now the 12" silvertone Cast grate,12.5 Cast grate and the 10" caste grate are all 19" long. The ironwood uses a grate that is just under 22" wide. So technically you can get the 10" and the 12.5 or the 12" silvertone and place them side by side to have a cast iron grate as the main grate. But keep in mind cast iron grate run hotter so it could effect your initial results.
I simply trough it on when needed, plus they are a coated cast iron so you are able to wash them with soap and water.