Go to sandbox on forge mode.

Go to moniter mode and fly upwards and look for a teleporter floating in the sky, and go throught it.

You will be further up, and will see a large grid spanning the whole sky below.

This can be used to place scenery, but going through it causes instant death.

Be sure to place extra teleporters because in slayer the floating ones cannot be reached.

Putting "walls" on he grid can act as a floor, and can be walked on.