This is my very own custom Skyblock map! I wanted to make one for myself that included everything I would want in such a world. Other skyblock maps are usually made with a normal world, carving a giant hole where the island is. This means you could technically reach natural generation if you travel far enough away from the orignal island. This world is completely empty, no natural generation anywhere! Other maps also usually have different types of biomes scattered around, this one doesn't. I don't know how to remove natural generation for the nether, so the world border has been used to prevent you from going far enough to see the natural generation. Don't worry, you still have plenty of space!
Every island and item is at X=0 and Z=0 in both the overworld and the nether. I have tried to build the nether in a way that prevents any shenanigans with nether portals. There is no way to reach the end dimension, but I have made it empty as well in case you want to add a way to get there.
I'll list everything that is included, but you can play it yourself to find everything. There shouldn't be anything that you wouldn't find by yourself.
A completely empty void (except for the islands) with only the default biome for all three dimensions
Overworld:
A 3x3x3 dirt and grass island with a birch tree
A jukebox
A chest with 1x lava bucket
The Plains biome
The Nether:
A line of 4x obsidian to prevent destroying the island when spawning a portal
A platform made of 4x warped nylium, 4x crimson nylium and 1x bone block
A chest with Pigstep
The Nether Wastes biome
Enjoy!