Please go here, it has what you want.
I'd go with the fiddle blocks
and 4:1 purchase. Remove the bottom end of the backstay, make a loop attach fiddle with jaw on bottom, and upper fiddle to newly made loop on existing backstay.
Note: Since the boats you are renting don't have adjustable backstay, please be very carefull as the mast might have "bending back" limitations, and you don't want to pay for it, do you?
Also, here is a simple fix, not as "stretch resistant" as a real backstay, but if the boat has a supplementary main halyard, (or similar) just tie it to the stern, as centered as possible, and use it as a backstay.
GO EASY ON IT, chances are those masts aren't prepared for that.
What boat is it?