It might be easier to change your wheel configuration than replacing all the boat hulls.
You’re only saving 4.5 kg per tile by going from sheet metal boat hulls to plastic boat hulls, so the difference is only 135 kg for a 30 tile vehicle or 270 kg for a larger, 6x10 vehicle. 135 kg is not going to make a difference unless the Sun Wave is powered by foot pedals. You’d get more weight reduction by swapping a pair of armored wheels for regular wheels.
Anyway, if you still need large quantities of plastic, I think there’s still a bug where laundry baskets determine their butcher quality by volume, not by mass, and thus produce 100+ plastic chunks per laundry basket. Trying looting a laundry mat.