You don't need an alternate light source, it'll be fine with just natural light. The room being pitch black at night is good as they need a day/night cycle.
I'm not sure about using Velcro so I'll let someone else answer that.
However, if you're using a 20gallon tank make sure to clutter it up a lot, loads of hides, fake plants and things like to make sure your snake feels secure.
Good luck for when you get your snake!
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