First, why are you getting a Magic 3.1? It is an old, outdated chip, with problematic code that won't play a whole slew of new games. You're really better off changing that to either the newer Magic chip, the Magic 5, or the Duo 2 Se, or even better yet, the Matrix Infinity.
20 watt iron is okay... just don't hold it down for very long on each contact point. The majority of us use 15W.
I'm partial to taping down my installs... but yes, many use hot glue. It's a bugger to remove though if need be. Tape, on the other hand... piece of cake.
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Oshawa modder... up there in Canada.