You can make a toy-ship out of a piece of wood. It will be bigger than a cortex boat.
Take a piece of wood-board and plane it so that the prow is pointed and the deck is smooth.
Make the keel (1) to help the toy-ship hold itself upright.
You can use a V-look twig for it (2). Put the keel into the bottom of the toy-ship. To do that drill two holes in the bottom of the board.
Fix the pegs of the keel into the holes. You have already done the mast and a rudder for a cortex boat.
If you take a bigger wood-board, the toy-ship will be larger.
It will be possible to set up not one, but two masts with sail or to fix an additional flagstaff (a pole for a flag).
You can use a piece of paper or rag to make the flag.