Plastic will survive park temps (200 degrees F) so in a pinch you can use a plastic container. Just heat your solution in a big stainless steel pot, run up to the max temp. Pour it over the rifle in the tank and let sit. By the time it cools off enough for the process to slow down it is done. Pre-heating the rifle in the oven will help too.
A piece of schedule 40 drain pipe with a cap glued on one end makes a nice vertical tank. Just make sure you pre-fill it to verify you have enough solution.