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Nice. It would be better to route the power cord through one of the other ports and use a strain relief. You could alternatively use a plastic grommet in the hole you drilled.
Tools and Materials for Arduino
The dowel technique works well with a rigid product like laminate counter or wood veneers, but something flimsy like a poster seems to need the paper mask to prevent accidental contact. Also with dowels you usually start in the center and work outward. The goal is to prevent bubbles. Starting at one end and towards the other should also work. I would use a window tint squeegee.