Do you have a pressurized tank? All modern cars that I'm familiar with don't have a pressurized tank but I know some older vehicles used a pressurized cooling system (I know earlier years of the Jeep Cherokee used it). If your tank just has a cheesy little plastic cap on it you'll know it's not pressurized. In that case I would think you could get away with taping it temporarily or a more permenant fix like was suggested with some JB weld. If it's a pressurized tank I would just go replace it.