Lash it on.
NOT a piece of (slippery)rope.
I would use heavy "seine twine".It is 3 strand,tarred nylon twine used in nets.Use a midsize.Not the thickest,not the thinnest...just the one that stretches a tiny amount with a strong pull.
Lash around the whole rung-not just thru those holes
but maybe start your first 3 turns thru them.
I wouldn't have put those holes in.Holes weaken.I'd have used maybe a screw head
to keep lashings from any very slight shifting on the slippery rung,but a good lashing won't move much along it!