Read Thread: Velcro advice
If you are making a cammo bag type of letterbox, use the soft part on the area where you zip it closed. It is rough to seal the bag with the hard part.
Thanks, everyone, for the advice. My box is a small tin, but I'm hoping to plant on a wooden surface, so I'm going to try tacking/nailing it down. I wouldn't have thought of that, but it's a great tip.