I've never lost anything out the top of my cheap folding grocery pannier, and have had much larger cargoes, and much smaller ones as well. Your basket is at the top of the "pendulum" where the bike sways back and forth as you pedal or change direction.... you need to get your load down lower, or move it to the front, or strap it in and hope for the best. Folding panniers, either cloth or wire, would work a lot better than a basket on top.
Wald's rigid wire panniers would let you rent out your bike to moving companies.
