Skyblock Builder
This mod allows you to create custom skyblock islands by configuring them through configuration files.
How It Works
To create an island, you need to prepare a schematic file using the standard structure block from vanilla Minecraft or the special Structure Saver item. In the schematic, you can specify possible player spawn points on the island.
Installation and Compatibility
The mod must be installed on both the server and client. When creating a new world in the client, you will find the "Skyblock" world type in the "More World Options" section.
For servers, use skyblockbuilder:custom_skyblock (in versions 1.18+: skyblockbuilder:skyblock) as the world type in the server.properties file or in the forge-common.toml configuration (this option was removed in version 1.19).
This mod requires the installation of LibX.

Slightly outdated, but may still be helpful
For more information, visit the Wiki linked above.
You are free to add this mod to any modpacks.