suemackey, this is great for you. However, the user who posted the question will not have such in their accommodation place. For sure he/she knows that safe facilities are available in other hotels/aparetments; however his/her vacation is already booked.
So the question is about alternatives.
Such were given by Toonman (the nest one under these circumstances) and also by Tillydrone.
I do fully agree that the solution of Tillydrone is not appropriate from the part of owner but from the part of guest who is not provided with basic safety facilities it is a matter of "belongings survival".
This I expressed above saying that one who cares for his ownership and wants to make the guests care for it also, should adequately care for guests' belongings by providing safety facilities.
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To franleeds (hope it's not late)
Rental safe deposit box is possible in banks and in main post offices. I don't think it's possible in SB post office but rather in the main one in Burgas.
Also, not sure if they offer so short term rental (assuming that your vacation is maximum 2 weeks).
Additionally, the procedure can take time.
Overall, safe deposit rental via bank/post does not seem neither reliable nor comfortable (unless they developed such special service for tourists, with which I'm not familiar and for which I'm definitely in doubt).
The best solution seems to be Toonmans' one. When I fall in such situation at my trips, I take the things with me or rely on padlocks.
The "curtain solution" is interesting but i'm a bit in doubt in level of safety it provides- I've been working on a balcony many times while the cleaning ladies make my room, and definitely wouldn't say that they do not mention some details.
Moreover, you must be sure you can after completely recover the curtain or just pay for "damaged property".