Tips for baking deep cake in ceramic bowl

I have to bake a doll dress shaped cake for my niece’s birthday this Saturday, but I don’t have a doll dress mold (and I live in a small town so won’t be able to get one in time) so I thought of baking it in a ceramic dish with the right shape. I tried it last night with all the tips and tricks I could find on the internet (flower nail in the centre, wet towel strips wrapped around the dish, baked it for an hour and a half at a lower temperature). When it came out of the oven it looked PERFECT! A skewer came out 100% clean and the top of the cake was the flattest I have ever seen. I even sent my mom a photo to brag with my “perfect” cake. I left it in the dish to cool and went to bed, only to find a disaster this morning. The centre had collapsed completely! I used a recipe for quite a dense cake and I’ve baked two normal 8 inch cakes from the same batter afterwards (it makes quite a large batch) which came out fine, so I know it’s not a problem with the recipe or the mixing. I know this is a very unideal situation, it being a very deep cake (about 6 in) to get the right shape as well as baking it in a ceramic dish, but unfortunately this is all I have available and I have to make it work! Any suggestions on what I can do differently? Should I bake it even longer, even when the skewer comes out clean?