I frequently head out on an errand run with a single pannier. It’s actually a smaller Ortlieb front pannier that I keep loaded with tools/pump and locks. More often than I would like I collect enough stuff on my ride that I can’t fit it into my single pannier so I have to get creative.
On this particular day it was getting warm so I took off my extra jacket that was necessary when I headed out into the chilly morning air hours earlier. I wrapped it around my extra items and used a couple small bungees to strap it to the top of my rack.
It wasn’t the most secure way to carry cargo, but it held together for the bumpy ride home.
I really should start out with 2 panniers, but then I have to carry them around when I lock up my bike which is a drag. My ad hock cargo solution may not be elegant, but it’s low hassle!