Here is the model I will be using. I painted this buy up a few years ago for the original BoLScon, and he will be getting a new coat of paint for War Games Con this year.
For this, I used a 3/16" by 1/16" magnet. You can buy magnets pretty much anywhere on the interwebs. The problem I have found with buying them is that cost of shipping is usually pretty. If I were you, I'd go in with some friends and then make sure you buy all of your magnets at once. You don't want to order, and then order again.
Now I will be using a dremel to drill out the holes for this, but my dremel will not take a 3/16" bit, so what I am doing instead is drilling a pilot hole (to make sure I am pretty much in the right spot) with a 3/64" bit in a pin vice. This hole will allow me to guide in the dremel. You need to be careful with the dremel as you can jack up your models pretty quick if you are not careful.
Once the pilot hole is drilled on both sides, I use the dremel to deepen and widen the holes. This can be a pretty tedius process because you are only using the dremel for a couple of seconds, and then checking to see how the fit is, and where you need to shave some more off.
Anywho, when you're done it outta look something like...
Usually when I have to drill something like this out, the hole is much deeper than the magnet is, so I usually have to full the hole with a little putty. Green stuff works great but I'm sure anything will work.
Just be careful to dry-fit the magnet with putty before you put any super glue in. TRUST ME you do not want to have too much putty in there so the magnet wont sit flush.
Here are some WiP pics.
After that you're all done. I'll be painting over the terminator, and while you'll be able to see where the magnet it, it hopefully wont be too much of an eyesore.
Next time we'll have some WiP of the rest of the terminators.