The solution is going on superflat and changing it to only 1 air block in the menu and setting bonus chest to on. Replace the chest with a block and continue on with whatever you want. But it only will work on creative, or you just fall out of the world.
theirongoldem if you wish to make a skyblock without regular world underneath, you need to choose creative mode and choose superflat world type, then you need to go to customize and then presets, where you need to remove all the numbers and letters, and then you type:
The 1 can be a number of your own choice because it determines the type of biome,
eg. 1 would be Plains biome whereas 5 would be Taiga Biome!
You must get MC edit and use //wand to cut off the srondings and leaveing a 7x7 (long) and 4x4 (big) Grass blocks...Then get a chest and put a lava bucket.1 water bucket.2 ice blocks and 5 tree saps(any)
Or without that MCedit it would be all most impostible!!!
Hope this helps Have fun makeing it! XD -KidCraftMC