Nice work.
I made a similar cargo carrier out of a small plastic tote, but it didn't lock. I just wanted something that would repel rain and make me not have to wear a backpack while riding.
Thanks, Money Monk. My "trunk" is also not fail-safe. It has a small lock and thieves can break that or even cut the zipties and take the whole box with them. It won't protect anything valuable, but I think it will be sufficient for grocery shopping and such.