I actually stumbled on a HBT user's post the other day that linked me to his build site. It is a copper CFC, but the first build instructions I've seen with copper in copper. He does a decent job of laying out the method.
HBT username is P-J
Here is his site: http://www.pjmuth.org/beerstuff/coun...ow_chiller.htm
Looks like he has built quite a few.
Also while doing my CFC research a great 8page thread/breakdown on the fittings etc.
I agree when using stainless the 3/8" inside 1/2" will be much less of an undertaking than something larger, however I have seen a couple brewers on here mention they would like to upgrade to 1/2" for the wort.
I would actually love a chillzilla as it has 1/2" inner tubing as well. But the $200 price tag makes me think I could probably do something DIY with stainless for cheaper (maybe more with fittings).
Perhaps with such hefty equipment you could look at making your own chillzillas with copper. 1/2" wort transfer benefit & I'll bet you would fall far below the $200.
And yes, if you end up making this, I would totally be interested in buying one