I'd recommend an SAE (the ones you see on battery tender lead) to USB adapter. If you want to take the extra step, you can also do SAE to Cigarette Lighter socket and a cigarette lighter socket to USB adapter. Of course, you'll have to be careful not to drain your bike battery.
There are lots of 10,000+ MAH battery banks out there so you can charge that using the SAE lead while you ride, then charge your laptop from the battery bank.
Solar panels small enough to fit in a tank bag are pretty inefficient.