A common approach is to lock the bike to something solid and to keep all your valuables like wallet and passport etc. in a handle bar bag that you always take with you. Don't worry about stuff you leave on the bike, 99.99% of the time it will be there when you get back, and if it isn't you'll have to get by without it until it can be replaced.
This also the way I usually secure by bike and gear.
However, I did fabricate security cables for my Ortlieb back and front roller panniers. They allow locking the panniers to the bike. I've only used them once in the 6-7 years since I made them; and that was in camp, not on the bike. Ortlieb has this option for their Packer series panniers, but not the Roller series. This may give you some ideas. They only prevent thefts of opportunity.
This may not work on the newer Panniers. We got our daughters Ortlieb back rollers for Christmas, and they have a different top mounting rail.
