I did investigate a solar roof panel for my SE.
1. Would have made a contoured panel. Yes, this can be done. A flat panel on a rack has an enormous amount of drag. I know this from carrying sheets of ply etc on low powered
ICE cars. It's like towing a parachute.
2. The L1 charging process has a parasitic draw and losses of a few hundred watts. This really, really hurts charge efficiency at low power.
3. The available charging rate. You cannot charge at all with less than 510W (6A, 85V). That's outside the specs for J1772.
You could use the solar panel to charge a small intermediate battery, then dump that in bursts into the car. This is the only way I can currently think to make it work. If I street parked without a charger, I'd be inclined to try that.
A ~1m2 roof panel could provide a few thousand miles of driving per year, depending on how much sun you get. Up to maybe 8-10 miles per day. But, it's something that is going to be very difficult to retrofit cleanly as it really needs it's own on onboard charging circuit. I'd happily pay extra to upgrade a moonroof car to a solar charge option, though. I suspect that there are quite a few low mileage drivers who find solar trickle charging a game changer. Think off grid, apartment dwellers and street parkers.
Others, like me, just wouldn't need to charge as often. I may just be able to commute with only weekend charging in the summer.