We just bought a box of Shimano HP-NX10 Handlebar Stoppers to mount onto bicycles doing extra heavy duty as front-loaded cargo bikes. $20/ea.
These come standard issue on Torker Cargo T and Batavus Personal Delivery bicycles.
They require a bit of extra steerer tube (1″ threaded, 30mm cup size head tube) to work, but they are awesome when you have to park your cargo bike and don’t want it to fall over. A perfect fit with a double-leg kick stand (which we also carry).