Hi, you cannot rent a safe anywhere.
I would just leave the money in the casa. You will see that the casas have bars on all the windows and door and are surrounded by a gate. Its very safe. And since Cuba is a cash society, I’m sure that landlord has her life savings in the house. The casas are very safe. I would leave the cash in the house. You are much more likely to lose it if you carry it with you.
Take care,
Mario