Waterproof fabric

Even if your bag does not need to be 100% waterproof - make sure the canvas you use is waterproofed, so you do not have to find all your wet stuff at the slightest dripping.
Many bags are equipped with a removable cover that you can be fixed in case of heavy shower.

Also make sure the fabric dries quickly to prevent it from staying wet and moldy. When looking for a thick but light canvas to wear, you'd better spill a glass of water on your bag and check if the inside remains dry.