Try using "EZ-LINE" which is like an elasticated lycra thread. Mine is a very dark grey and is a couple of years old so they may have changed the colour since then. It stretches to several times it's (cut off) length so you only need a small length; for a 54mm fig perhaps 1" os so. Secure it at the one end with a minute drop of superglue (it will 'hold' almost immediately). Give it a few minutes to fuly bond then using some fine tweezers pull the other end into shape over the hand/fingers as necessary then secure likewise at the other end once you are happy with the correct bowstring tension.
Do a couple of dry runs before gluing the second part and also don't cut the initial length too short so you are having to stretch the line to it's full extent. It will keep it's tension and seems a very robust product. I still have it on model a/c etc built years ago where it was used as aerials or control cables and no fraying or reduction in elasticity is present. Some other brand of stretch lycra thread may yeild a similar result if you cannot get some EZ-LINE?
It will also take very thinned paint (I have used waterbased acrylics)if you can only get hold of clear or coloured lycra threads.
When everything is fully set (leave it a day or so for the superglue to cure)just touch in with paint the tiny amounts of superglue that secured the thread or else you may get the 'glint' from the glue as the light catches it.
In 54mm the bowstring will look 'tight' and also look in scale.
Others however may have alternative and more effective techniques?
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