How can I spawn an arena of walls around the portal and change item ingredients?

[Ninjasmoker]

1. I want an indestructible arena made of moonrock-walls around the portal on my dedi server when world generates, with 2 entries. How can I do this?
2. How can I change the ingredients of an existing item? I tried this:

_G = GLOBAL
Ingredient = _G.Ingredient

if book_sleep then
                book_sleep.ingredients = {Ingredient("papyrus", 4), Ingredient("nightmarefuel", 4), Ingredient("goldnugget", 2)}
        end

or with the table_insert command:

table.insert(_G.AllRecipes.book_sleep.ingredients, Ingredient("goldnugget", 2))
                end

Thank you very much for your help!