The secret to all my carts is the platform. Once a strong platform is made, you can add any kind of basket or sides you would like. Take a look at the three carts, get an idea, and then go with it.
I use 2 inches by 2 inches wood to make the frame. I always add the center supports, because these carts have to carry a lot of weight. Once I have the frame built, I add one-quarter inch plywood base. And then the sides are up to your imagination. You could simply mount a basket to really make it easy.
I get the tires from Northern Tool Company, www.northerntool.com. They cost about $20 each. The axel is a rod of one-half inch hardened steel from a hardware store.