You could always splice an eye into your rope halyard.
You could make a continues loop of dyneema, aobut 9" diameter and tie it like a Prusik not so it grips the halyard.
Here's an image from
Wikipedia:
To increase holding power, you could give it an extra turn. Assuming you can get a firm enough grip with in you your halyard, you would hook your tensioner into the loop (the bit that extends below in the photo).