Put a Zip tie round your case (through two zips together) then you can see if it's been opened straight away, and incase they need to see inside it for something they can cut it without having to damage the case.
I try to never leave any valuables in my main case though as I don't trust anyone.
when you travel to america you can get combination locks that customs can open with a key, is there no such lock you can use in other countries?