If you are going to be wiring things up on your own and not buying commercial products, I would have probably wanted to say that it's a good idea that you study up a bit before risking your camera. (You haven't disclosed the situation/environment under which all this will be operating -- except perhaps that you are powering your controller already from the camera itself.) But the fact is, you've already gone off and taken risks just powering your controller through a regulator directly off of the camera battery system. So that's water already under the bridge now.
I suppose if you are already doing that much and accepting the associated risks that already attend hooking up something custom to the camera power supply, then you probably don't need to waste time protecting the shutter trigger. Just use a cheap BJT or NFET as a switch and be done with it. You could go the extra mile with an opto, which really is just a BJT at the receive side anyway, but I don't see a reason why you'd care to do that if you already powering your controller using the camera power supply. You are galvanically connected, already. So what harm would be barely a little more?
If you decide to power your controller separately, though, then it might be worth your time to also go the relatively easy extra mile needed to galvanically isolate your shutter triggering connections, too. Then I think you should consider the opto. These come as either opto-BJT or opto-FET (for example, the Fairchild H11F1M.) It's not hard, either way. (You could also consider a reed relay if you can accept its slower response function and you are willing to build a circuit for it. But the opto is cheaper and easy, so I'm not sure why a reed relay would be used here.)
But with you already picking off power and hooking up a regulator circuit, as it is now? Just a resistor+BJT or logic level NFET can probably do the job without additional risk. You've already accepted the risks.
There must be hundreds of web sites on this topic for DIY folks. Have you not tried to read any of them?