pick any popular modern hmpe string fiber and have at it. almost all will be around 100#/strand tensile strength if not more, so they're fine for low strand count 'skinny' strings. i prefer the bcy fibers, such as dyneema'02 and formula 8125.
as to the center serving diameter, that'll take some experimenting/testing, but in a pinch an added underlying or overlying serving of dental floss or string fiber will help to build up the nock point area of the center serving. don't forget to pad the loops!