I've decided to stop building these, as it's more trouble for me than it's worth. As requested, I'm re-posting the schematic for my EMU-triggered O2 clamp here so that everyone can build their own if they want one. Perhaps some kind mod will sticky this so people can find it more easily in the future.
The resistor values are not absolutely critical. If you have to sub a 4.7k instead of a 4k it's no big deal- it'll just shift the centerpoint on the potentiometer a little. A note about the pot itself, I much prefer to use the more expensive box-shaped 15 or 20 turn units as opposed to the cheap single-turn units. You need a lot of precision when setting this resistor, and a multi-turn makes it a lot easier to set.
The diode across the relay can be damn near anything that isn't a Zener. I've been using 1N4004s simply because I have a pile of them. Use something that has a 1N400x
number and you'll be fine.
The regulator probably doesn't need a heat-sink. It's only passing a few ma.
The trigger output of the EMU is 12v. Since I'd been using 5v relays (again, I had a few extras) I had to put a resistor in series with the coil. If you use a relay rated for 12v you can jumper across this resistor.
Instructions for setting up the EMU are here: http://miatajoe.50megs.com/O2_Clamp.pdf