Something like this maybe:

https://smile.amazon.com/dp/B000KZXJF8/ref=cm_sw_r_cp_apa_glt_fabc_9TTPKBVEDZ4X684B68G1

I doubt there is much that would be too secure. Any lock at best would be a deterrent, so keep that in mind with what you plan to store. Anyone who really wants access, likely could get inside without issue.

It raises an interesting question, would a locked door motivate a bad guest to go searching for things moreso than an unlocked closet full of the host's belongings that was simply out of the way? I'd probably go with the lock, but interesting case study perhaps.