SkiffRopes sell unfinished 18 foot skiff tapered spinnaker halyards for $180.
You get 12m x 6mm LIROS Magic Pro joined and tapered into 12m x 3mm LIROS D-Pro.
You need to finish the halyard length to suit your skiff. You can trim each end to make sure that the taper sits just in front of your cleat (i.e. 500mm). There is plenty of 3mm D-Pro in the tip that you can bury to make a tip double.
You should wash your halyard before use to reduce how slippery it is.