My stove is the same as yours. I used to cut 18", but found that I still had problems if there was too much accumulation at the back. It also made stacking on pallets complicated and inefficient. I gave up and started cutting 16", which always fits the stove and a 48" pallet. I use a Tap N Cut, and have made a few for friends after they saw mine. Painted orange to reduce the chances of losing it.
Like others said 2” less than maximum capacity. Gives room for airflow and a little margin of error when cutting if your an inch off it’s no big deal. Looks like 20” would work real good. My insert is 20” so I cut to 18”.
Yeah we replaced the top once and it needs it again soon. It’s neat the whole thing is just bolted and pinned together. No idea on manufacturer. With the government likely whoever could do it cheapest.