surface area is what will increase your humidity. the wet towel maybe doing more for your humidity than your container of water, if it is not wide enough.
for instance, our incubator has 3 trays. the front 2 smaller trays filled with water will get us to 50%. filling the larger back tray with the front to will put us at 70%.
try a large, shallow pan with the fan blowing across it, if possible, and make note of your humidity level