Steve, am I missing summat? Shouldn't the seats fit over the raised lip on the seat base rather than just sit on the seal. The rebate around the top of the grp base should bear the weight of the seat & your fat arse body & the seal should just touch against the underside of the seat base. In theory you shouldn't need a seal as you form a labyrinth with the seat, if that makes sense. It looks to me that the 2 lateral bits of wood under the seat need shortening.
Rule#2: Never argue with an idiot. He'll drag you down to his level & then beat you with experience.
Rule#3: Tha' can't educate pork.
Rule#4: See rule#5