Well, I didn't intend for people to be able to "pick locks", but being able to kitbash things like that is one of the cool parts of the system!
For unspecified environment things, I think I would just go with d8/d8 – a "middle of the road" difficulty. That's not supported in the rules document currently, so maybe I need to add that as a sort of catch-all.
Edit: Don't forget to assign traits to those for storytelling purposes! Try d8 Rusty Lock / d8 Noisy Tumblers or something.
I'm glad you're enjoying the game! I'm hoping to steal some time to extend it beyond the original release sometime soon!