i d say that you will probably be making more than one waterblock. so i would go to
mcmaster-carr.com and buy an o-ring cord kit so you can make your own custom sizes.
and i would steer away from "clear" tops as they have a tendency to crack or leak over time.
just make a solid block out of copper. or make the top out of aluminum and either anodize it or do something to make it non conductive so the battery affect doesnt occur from mixing metals. but bottom line is that you are going to have to put something to keep it watertight, and an o-ring is the best way to go
