From the pictures that were already posted and that I have read previously that the plastic casing for the GF is already established(read: injection molding complete), there were no locks on the lid. Plus, kids are going to think its cool as heck. Children with small fingers, despite being told DON’T PUT YOUR FINGERS IN THERE, will almost certainly stick thier hand in the pass through slot when no authority figure is around to see why, and because it has a cool slot. You might also find small toys, pennies, or crayons put into that slot if its not locked down and covered. I have a 2 year old. The internet is rife of parenting stories. I’m not taking any chances either, when it comes to this.
I think your box idea is pretty solid. Just bolt or screw in a shed door hinge lock or something on 2 opposite sides.
There was a discussion about how to transport, and the overall consensus was that there’s a company that makes sturdy metal cases that have high density foam linings, that would protect sensitive equipment like the GF while transport. As I recall, most of those cases were set to lock, or could be made to have a lock. Might be a few hundred bucks, but in addition to storage it also allows for transport.