When you put the felt on the chassis, make sure the seat belt mountings to the centre of the chassis line up with the holes on the fibreglass shell.
I had too thick, and I had to compress the shell on the chassis using G clamps on the hole in the middle.
Too thin, and you have to jack up the shell from underneath to get the holes to line up.
Best to do this as soon as the shell goes on the chassis, so you can easily adjust thickness of felt if needed.