I didn't actually catch that you were thinking of mounting the panel under the glass. Not sure that would work so well (but it's good to consider things outside the box).
I've had the fridge in the car for a couple of years now. First just only running it when driving via the Forester system, and not too long after with my separate house/solar system. For the way I use the car, it's hard to now imagine not having it. Camping of course, but also grocery shopping in the city (hours away) and being able to pick up ice cream, coming back to the car from kayaking or hiking to cold beverages and food, road trips, and just being able to keep a "stock" of food in the car so I don't always have to eat out or only eat Clif bars (not that I don't still eat Clif bars).
I put in the battery setup before I removed the rear sseat, so that's not required. It lives in the passenger side rear footwell (obvs not a fit if you carry four adult passengers).
The battery is only a Group 24 (100 watt solar panel), so it can't go days without sunshine, but of course I can always turn it off, and/or maximize solar charting by running it off the car system when driving. And even without any of that you can run it while driving of course. Once caveat is that these modern compressor refrigerators don't like voltage drop. So the rear outlet is pretty much a no-no unless you re-wire it (Subaru wire too long and skinny). When I do use the car's system, I use one of the front outlets, and also I changed the refrigerator cord itself to larger wire, to help make up for the Subaru wire.
But for probably 90% of the time it just chuckles away on the "house" system (I don't garage the car tho, and I'm usually in a fairly sunny place).
As I'm sure you can tell, I really like having the 'fridge in the car.
BTW, thanks for all your posts on the Yokohama GO15 tires. I've been lucky with various highway tires up till now on rocky desert "gravel" roads, but I think it's time for something a little more durable. I really like the idea of 16" wheels and slightly more sidewall, but wheels add considerably to the cost so I may go with the 225/65-R17 on my stock rims since it sounds like they have worked well for you.