Flat battery several times due to infrequent vehicle use over several weeks. Charging is a pain. I'm looking at small panels for battery maintenance. I can live with connecting and disconnecting a panel. I'd like it as easy as possible. I'd prefer it to remain connected while driving and forget about it.
Some panels come with a cigarette lighter plug. I doubt that technique really works in a modern car?
Internal suction cup mount is common. I was originally thinking of external gutter mount. There's nothing like that available and I'm not sure I'm keen on the cable routing required.