On Sat, 04 Apr 2020 22:07:13 +0100, Chris Hogg wrote:

snip

Not sure I see exactly what you're looking for, but look at these -

Garda hitch (needs karabiners but you may be able to improvise)
https://en.wikipedia.org/wiki/Garda_hitch

or Trucker's hitch https://www.animatedknots.com/truckers-hitch-knot ?

If it's to be able to slide the knot when it's not under tension then
a Prussik might do the job?

https://en.wikipedia.org/wiki/Prusik

No karabiners required. ;-)

Cheers, T i m