You wouldn't want it made from aluminum as most watercooling components are copper or brass...aluminum will react with these metals causing galvanic corrosion unless you run some sort of coolant, like glycol to slow this process.
There have been people that have done this before, but it does become cost prohibitive. It is a neat idea, but getting a custom made block that has the performance of a block that is manufactured is a real struggle. Either way, it's a really neat project if you have the time, money and tools to do it.
Have you Googled around the web much to see what others have done?