We(that being my husband and I) made the square parts in an adjustable square cake tin, and then used cardboard bent into a curved shape and then covered with foil for the top part of the pirate treasure chest birthday cake. Chocolate fudge filling to over it and liquorice laces to be the black bits.
The ‘treasure’ was midget gems and chocolate money. The map was made of rice paper so all was edible apart from the foil over the coins. The sand was light brown soft sugar. The key hole was made of rollout icing and painted with edible gold paint. Along with the cake there was an hilarious pirate party which the adults seemed to enjoy even more than the children!