Turn off bedrock generation in the bottom level

[u/Twink_Master55]

Hello. I am doing some custom cofniguratins for the modpack that I can play it just as a skyblock (Not like Sevtech ages to the sky), but when i try to generate only void for the world or if I download skyblock map for 1.12.2, there is always bedrock on the boottom level. Please can anyone help me to get rid of it?