Cooler probe

I need the attached part made. This is part of a cooler and will be fitted with a peltier device on the back it needs to have good thermal conducting properties in water such as aluminium. It is going into a freshwater aquarium and needs to be water resistant and not leach any chemicals into the water/react with water. Can you suggest a material and/or coating suitable for this application? Only the cylindrical part needs to be coated if required. £30 is my budget for this project. Is this do able? Is there a better/cheaper way to make this? http://www.docdroid.net/132ev/probe-1.pdf.html

Thank you

Daniel.