As Doug shows, simply using bungees is a simple option.
A little note of warning, if you do go this route (49L and 70L panniers) be wary of the rack pack being overly heavy. When I've put heavier items in mine, it did make the bike a bit waggy waggy simply because the weight is so high.
Kinda like when people attach a milk crates onto their rack and put heavy stuff in it.
I learned to put heavier things low in panniers and bulky but lighter stuff in rack pack.

But frankly, with 120 litres of stuff plus your weight mostly on the rear wheel, be prepared for a possible crapstorm of spoke issues that this setup could bring.