This mod allows you to create custom skyblock islands via config.
You simply create a schematic file using the structure block from vanilla Minecraft, or the Structure Saver item and set possible spawn points for the island.
This mod is required on server and client side. It adds a "Skyblock" world type at "More World Options..." setting when creating a new world on client side.
For servers, use "skyblockbuilder:custom_skyblock" (1.18+: "skyblockbuilder:skyblock") as world type in server.properties or in forge-common.toml (config).
This mod requires LibX.
Help translating the mod here on One Sky.
For more information, please visit the Wiki linked above.
You're all allowed to put it in any modpack.