Preventing Cats from running outdoors

I just got two new cats, and since I'm in the Northeast they're restricted to the inside because of coyotes, etc. I'm looking for sure fire ways to make sure they don't get out, specifically out of the garage.

It's easy to watch the side doors, and they don't seem inclined to want to leave anyway---however the litter box is in the garage, (they have a cat door to get into the garage from the house). Since we got them a couple of weeks ago, we've just disabled the electric garage door opener and make sure it's safe to open it, the open it manually (even with the car parked outside we still need to go in and out a lot through the big door).

I would prefer to have the garage back at some point, there's no chance of the cats getting run over, I'm more worried that one of them will try to bolt out the big door.

Are there any highly recommended products or methods for keeping the cats out of that part, something like invisible fence, but without the collars? I've seen stuff on the internet but am reluctant to try something and find out the hard way it might not work. Any recommendations would be appreciated. Thanks

The first thing you need to do is stop letting them in the garage - get rid of the cat door. Find places around the house for those litter boxes. As for seeing and smelling the boxes - the rest of us live with this just fine.
Once you do that you can start on making the garage and that door an off limit zone for the cats. Clap your hands, stomp, and go pssst when they get close to it. You can also get motion activated devices to make noise but you would also trip these.

To be honest if you clean the boxes daily they shouldn't stink anyways. Since I've been pregnant we invested in one of those automatic ones that scoop 10 minutes after they've done their business. We also add some of that arm and hammer stuff just in case. We keep out litter tray in the bathroom.

As for preventing them for going outside, if you succeed let me know! Winston and Murphy are indoor cats. Now Murphy is fine, he has a total emotional breakdown at even a hint of going outside.

Winston however is a different matter. He .. barks (yes barks) at the door to go out and a couple of months ago when it snowed the hubby opened the front door to look out, out bolted Winston right in to the snow, shortly followed by his pregnant owner in her bare feet and nighty trying to get him. I'm glad it was early and the rest of the world was in bed haha.

Good luck!