Two years ago, we had a discussion involving the slippery 8 and a very good variation thereof, the so-called TALK knot AKA Quick 8.
See
https://igkt.net/sm/index.php?topic=5247.0 for details.
The original poster used the TALK knot to make adjustable loops for kites, and if it was good enough for that purpose I'm sure it
will hold the strings of a wind harp taut with ease.
The Quick 8 or TALK knot is simply a Slippery Eight with the tail end reeved through the figure eight knot from the other direction.