When I picked up my build, I found that the fiberglass people had been very generous with the flanges. I determined that 3/4″ flange was a good place to start, it is unlikely one would need a longer flange.
I cut a short piece of wood, making it 3/4″ thick and about an inch long. I radiused one edge so I could put it on the flange and use a marker on the outside to generate a trim line
You’ll notice the pile of trimmings – and this only represents half of what was cut off. In other pictures on this site, you’ll see wide strips that were cut off.
I found a Sabre saw or small sawzall with a carbide grit blade did the best job. They need to be a longer length since the saw base is riding on the outer surface which is about an inch away. Be prepared, I went thru three blades. The fiberglass is VERY abrasive
