If you are planting where you get freezing weather, you won't be able to get the box free. The velcro gets water in it and then freezes. I found a box here in NY and when I went to pull the box free the velcro ripped away from the bottom of the bench, but was still attached to the box. I now had a box that wouldn't reattach to the bottom of the bench, so I hid it in the woods nearby and notified the planter.

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.