I store my custom panels on the roof on edge as that was the only space left, behind the second spare wheel to cover the leading panel from bugs. 

But I do take them off the roof if we are on corrugated roads as all up there is 142kg up top. Not to protect the panels but to reduce the stress on the car. 75kg including roof rack should be the maximum up there!

I would just make sure they don't fly around & nothing will fly into the panels. If I have the panels sitting in the car I have them sitting on a bath towel not on edge as this is simply the easiest way, & clamped down.

The way portable folding panels are put together always baffled me. The glass is the most fragile part, so why do they fold with the glass on the outside? We pulled ours apart and put them together the other way, so the glass panel faced each other. Then we added a piece of closed cell foam cut to the size that fits in the back frame of the panel. Now the panel is protected from things hitting against it and smashing the glass.
Now you can store them on their size with a strap to stop them becoming a projectile in an accident or sudden stop, or on the bed with a pillow to stop it launching at you or wrapped in the doona to keep it in place.
The important parts are, protect the glass from something hitting it and shattering it, and stopping it becoming a projectile in an emergency

T1 Terry

Same panel just lays flat on top of the bed while travelling its clean and does not move an inch,when parked up its out in the sun,and sits standing up in the car back seat ares of a night,when in caravan parks its stowed neartoy under the bed.