I don't normally ride while wearing a shirt & bow tie, but here is a photo of one of my solutions. (I'm heading out on the DGR ride).
It's a home made top-box which is locked to the bike and water tight and lockable.
I don't ride with a pillion so prefer not to hang stuff off the rear of the bike. Personal preference, of course.
If I need to carry larger volumes, I use a big rectangular hold all. It's not water proof, so I put things in separate plastic bags. Works ok, but not ideal for long tours of course. The main benefit is carrying capacity. (The OEM panniers are too small in my opinion).