Help with Barking at Things while Driving?

I have a 7 month Poodle puppy who is reactive when she sees other dogs. We've been working on it during walks with engage-disengage and desensitization training, but I'm at a loss for what to do when driving.

I'm a solo owner, so I don't have an extra person to distract her while driving. And I don't have many options for places to park and practice with her. I've tried local dog parks, but all of them have parking too close that she's immediately over threshold. There's also times she starts reacting, and I can't correct or distract because we're going down a major road at 40mph (~64kph) while on the way to a park or something.

Finally, she's too big for a crate in the car (I'd have to buy a bigger car). I'm getting a car harness, but am still waiting for fully growth since the harnesses are ~$200. Plus that just stops any physical movement, rather than helping with desensitization training.