The ideal lock box shouldn't look like one.
Only a decoy should look like a lockbox.
Make a box that is impossible to open. Something from
steel, with a fake lid, welded shut from the inside with a
big lock on the outside. Now bolt the decoy to the frame
where it will be found easily, maybe under the seat. Use some
wimpy fasteners, so it can be torn out, without damage to
the bus. Then hide your stuff somewhere else, and let them
bite their teeth out on that thing.

I once met some guys from London driving a nice looking
74 camper bus on the Greek isles. The odd thing about that bus
was that it had these gigantic padlocks welded on the outside to
every door and hatch. Even the engine hood had one.
Never seen anything so ugly on such a nice bus.
But I guess it worked quite well, especially as a deterrent.
Basically they turned the whole bus into a lock box.

Another deterrent is to install a red blinking light somewhere
by the dash. Hook it up to a small 6 volt flashlight battery.
Blinking red lights work on car thieves like garlic on
vampires. But whatever one does, anything of value
left in the vehicle is about as safe as if it's laying on the
street.
Harald
90 westy