flush) and coat with fiberglass, similar to how a surfboard is made. Maybe a better question is how many do you need and what is your budget?

My imagination tells me this is for a particular petroleum brand. In which case fiberglass is probably your best bet, since this will be living outdoors and have some amount of kids wanting to climb on it. You could possibly 3D print a mold, in sections, and bolt it together. Coat or treat the mold halves (epoxy? Ask Smooth-On) and do a gel coat+fiberglass layup. A chopped fiber, fiberglass lay up is probably your best bet but look into how boat hulls are made.