Unless its a toy radio controlled model S then a portable solar generator is going to be a rather slow way of charging. This Solarpod Pro | Thousand Suns Solarpod - the portable solar generator is 100W so if you are lucky you might get 1kWh per day so about 3 months to charge?
You need to be able to generate at least 1.3 kW @110V continuous in order for the car to charge. Also the Roadster is well-known for being fussy about its power source - most generators do not work. I'm sure the Model S is similarly fussy. So the odds of this being successful are low.
Although unrealistic, a solution like this may be something close to what are you looking for? Although it is rated for 19kWh/day, if it were fully charged and you plugged in for the day you could conceivably get 38kWh in a day. But is ability to really be mobile, as in with your car, is not realistic. http://www.mobilesolarpower.net/ms_series/ms-375/