In reply to neuromancer:
An overhand knot on the bight is OK to make the loop for hitching to the harness. It's what I've used for the lanyard I have with a Kong Slyde.
It's marginally weaker than a fig-8, but the normal downside of being far harder to untie once loaded isn't an issue in this scenario. There's probably slightly less shock absorption than a fig-8 but any dynamic lanyard will be far, far better than the static alternatives. Cross loaded, overhand knots can still slip (although less than a fig-8) so it's probably worth pre-loading the knot and avoid having a really short tail.
As others have said, used a barrel knot for attaching a karabiner to the other end. (Although the better option might be to make it adjustable by buying a Kong Slyde - https://www.decathlon.co.uk/kong-slyde-id_8388518.html assuming you can actually find one in stock somewhere.)