I found a source for a relay board that works exactly like drawing GP3. It operates on 12 VDC and has all screw terminals for easy connection. Tried it with my GPUSB (which Doug nicely repaired after my previous experiment) and it works great.
Here's a link to the page:
http://www.simplecircuitboards.com/1A%20Relays.html
The owner is Jeff Wheat and he make 2 types of boards: sourcing and sinking. Only the sourcing boards are displayed. The GPUSB works only with sinking boards, so you'll have to ask for those as a special. The price is the same. He'll also modify it for different voltages for almost no cost as long as he doesn't have to change his design too much.
What's good about these board is that Jeff tests each individually (so you know they work) and they isolate the GPUSB from the outside world. Great time saver!