2-8, I don't have a FXT, but you can get a good idea if the tube diameter with a tape measure or ruler. Just lay either across the tube and try to sight the outer edges. If you had a dial caliper, you could get an exact measurement. When I ordered the rear view for my 2020 mule sx, I found one from side by side atv. It came with several different plastic clamps for different size tubes. I still had to wrap the mounting area with some electrical tape to take up the slack but it works. On my '04 mule 550, I had a bolt on mirror so I drilled a hole in the top tube and got a longer mounting bolt and it works great.
Hope this helps,
Roger