Cathar, I don't think you should be recommending tap water. I don't know what the tap water is like in Australia, but in many places here the water is chock full of calcium and magnesium, as well as small amounts of yummy toxic stuff. Washing cars is a PITA, because the water is so hard it leaves lots of thick water spots as soon as it dries. These mineral deposits can, over time, form nasty green lime scales, much like this:
This is a block opened up after 6 months of running tap water, with no corrosion inhibitors. This PC was powered down every night, so the time the water was left stagnant might have something to do with it. As you can see, the channels are getting constricted by the deposits. Very little crud is from actual galvanic corrosion; only the thin black layer on the Al and Cu is caused by that.
CLR is not recommended for use on Cu. Read the label. It will get rid of lime scales, but probably corrode the Cu
and take the anodizing right off the Al. And if your block top is already corroded, only re-anodizing the top will repair it.
I don't know why people still insist on making Al tops for copper blocks... Lexan is not only stronger but you can visibly inspect blocks for crud without removing the top. Even if an Al top is Type III anodized, a small scratch could prove disastrous. I would bet these scratches are not hard to get, especially when screwing down metal barbs. Anodizing alters the thread pattern and diameter slightly, and when that metal barb is screwed into a too-small thread, something has to give.
Anyway, Cathar, I know the new WW blocks are going to be mixed-metals, so I would like to hear your rebuttal.
pH: An acrylic block might work nicely for your purposes. You can get a 3x3x3 block for around $30. Here is just one place that sells them:
http://www.nicetoys.com/jaextralargeacrylicblocks.html . Just use poly barbs and be careful not to crack it when machining it. I think you'd be hard-pressed to find Lexan in that size.