Eventually you need to open "something" to let the dog out, whether it be a gate or door, so you're going to have an issue either way. I just stand behind the hatch and open it while I'm there. I can block/grab the dog if she wants to jump out. Though, having two pups I bet this would be a lot harder. But you're going to need to let them out of something eventually, so you'll have the issue no matter what.
I've been working on impulse control with my pup since she was 10 weeks, something I'm proud of, so she has a solid "wait" command that I can be pretty reliant on, but even so, if we're in a high traffic area, I won't open the hatch until I'm standing at the back of the car.
The other thing you can do is to just keep the dogs leash on them and attach it to an anchor point on the back of the rear seats, so if you open the hatch, they're still attached to something.
With that "hack" you really can't use the back seats. You'd still need to open your back door to then open the gate, so there's still the risk of them squeezing out. Aussies can be pretty quick and squirmy
To be honest, reaching over to leash them while they're in the "hatch" from the back door is going to be a pain in the ass.
A variogate won't fit either (I've tried to measure it).
I still think the best bet is just going with a VarioBarrier HR, and if you're in a high risk area, just climb into the back seats (like you would anyway if you did the hack) and leash them up from inside first, attach to the barrier or anchor point, and then open the hatch. Then you can grab the leashes, and unlatch them when they're secured in your hand.