I built my twin USB cable release using a big LEGO plate and a couple of inexpensive parts from McMaster-Carr: a cheap 9v battery holder and a nice 6mm panel cutout switch. I did a very small amount of very poor soldering, but mostly the wires are connected with pigtails that I've protected in heat-shrink tubes.
My little cable release working well, but I'm going to be improving it by swapping the two micro-USB cables out for lower-profile angled connectors and making a smaller, more elegant, and more robust enclosure.
Long story short: This was a really easy little build, and my two
Canon A4000IS cameras were cheap and work great. Thank you everyone for the great ideas for USB cable release specifically and for the awesome CHDK software generally!