Such a cool mod. Nice write up.
How much does this end up costing when done, looks like <$100.
i ended up paying $85 shipped for the core, $20 for the heavy gauge wire (used, from someone's amp install) and then the misc hardware and fuse.
if you use the connector and repair wire that probably adds another $20 which is why i am recycling the wires off an old pc beeper speaker or something (it fits, mostly).
I also already have a hammer-type crimper for large lugs. High current wires should be crimped well enough that you can't pull the lug off. if you have a blowtorch or an exceptionally powerful soldering iron you can solder them too, but current spikes can cause the solder to briefly melt which can lead to a fire if you did not also crimp the connector on.
hammer-type crimpers vary from $15 on up. I have this one i bought for my dual battery & winch installation in my land cruiser:
http://www.amazon.com/gp/product/B003X51S00/
I should stress, again, that if you use car-part.com to find it like i did, you do need to actually talk to someone and make sure you're getting the electric core and that they're really pulling it out of a tdi. The first guy i talked to was all set to try and dig one out of a gasser A3 for me, which wasn't going to work, and he wasn't following what i was trying to tell him. So i called the next guy.
Another thought: 10 feet of 4ga is more than enough. 9 feet would do it in a pinch, i don't think the battery lead needs to be 6 feet long. but I'm not sure 5 feet would do it. just saying, if you come across a good deal on not quite 10 feet, it'll probably work out with the same kind of 60/40 split. I had plenty of slack, and i used a few inches less than 10 feet.
If you're made of money, i'm pretty sure NAPA and some other auto parts stores can make you custom battery cables to spec. 1/4" being 6.35mm, ask for 1.4" lugs.
I bent the negative lug on the heater core end into a Z shape with vise & hammer. right-angle lugs exist, or you can just bend it into an L yourself.