I hope you won't mind me posting this; it's meant in the best of intentions
If you've just had knee surgery of any description and you intend to carry 80 plus L., I suggest you invest in a good pair of walking poles and learn how to use them properly; my orthopeadic consultant has told me that without doubt they're the best thing you can ever do for your body in general and your knees in particular. He says that it's beyond doubt that used properly they'll take 30% of the load and be of major benefit.
Of course being an educated and wise member of this august forum, you probably know all this