How to add nether ceiling to normal world gen dimension

Hello modders. I need help with my dimension that is "under" the normal overworld. To achieve the feeling that it really is under the world I want bedrock ceiling above the dimension like in bedrock. Hovewer, the dimension is generated like normal overworld and I am not able to make it generate here. I even tried to copy nether-like dimension ceiling code but it does not work. If you have already tried this or know what to do, please answer here. I'd really appreciate that! Thank you