I know how to do this.
You do the flaming red hovercraft, get it above the invisable roof, then put the hovercraft on top of one of the mountain and then get out of the hovercraft but make sure the surface is level first or it will slide of, once you done that repair ther hovercraft so that you have a normal one then get back in. Because the hovercraft is no longer destroyed it stays above the invisable roof, even if you try to make it fall, it will stay above the invisable roof.
Thats how you do it, I dont know how to walk on the invisable roof yet though.
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