SEAT
I'm guessing somewhere around 3/16 to a 1/4" heck the ones I have replaced they disintegrate in your hands !   I buy the large plastic mud flaps that are used for Semi trucks and cut a bottom from these that will never rot !  They are inexspensive ~$12.00 and you can get 2 from each flap.   At least use linseed oil on both sides if your use plywood.
			
			
									
						
							https://www.youtube.com/watch?v=PFb6NU1giRA
"I prefer dangerous freedom over peaceful slavery." Thomas Jefferson
			
						"I prefer dangerous freedom over peaceful slavery." Thomas Jefferson