I did a carved Thomas the Train cake for my son's birthday. It turned out great due in part to advise I found here on carving cakes. Do a search but the top tips I found were to use a relatively dense cake and freeze the cake then carve it frozen. Another 3-d cake I made was a frog - I used the Wilton Stand-up Bear pan as the base and modified the shape by carving it and icing it. Perhaps that might also work as a base for TMNT? Good luck and have fun.