Minecraft Skyblock building down in nether

You have 3 options in my opinion:

1)Acquire a piston. With this piston you can push blocks down. This can be repeated as many times as necessary. (Iron can be obtained by waiting for a iron bar drop from a zombie in a mob-spawner.)

2)Have a watercourse near your over-world portal. Create a down flowing lava-fall and drop down in this to build. Quickly return to the over-world and put out the fire. Rinse and repeat.

3)Plunging to your death whilst placing blocks on the lowest level of your island.

simple solution: piston pushing the cobblestone down.

less simple solution requires the witch to throw fire resistance on herself, you being affected because of standing near her, than running through the portal and lava-scaffolding.