@Prototype115r the main grill surface of the RecTec BFG pellet grill is 23" deep. The sear kit you linked above is for the Rec Tec Bull/Stampede which are 19.5" deep.
Technically, you could use the Bull sear kit on a BFG just leave your existing wire grates in the BFG. In face, I have a Rec Tec Bull and the sear kit but still leave my stock grates in the grill... call me lazy I guess.
Grill Grates does offer custom sizes, but sadly the closest they offer to fit the grill as closely as possible is 20" inches, so you'll still need to keep the stock BFG grates in the grill.
Call grill grates they do custom ones they pointed me in the direction of the 20 in they have and told me i would need 4 to get u half of the surface of bfg they told Rectec has not reached out to them but if they did they would recommend those