How about a BMF light sail?
If you flew out through the south solar pole magnetic field, you could get a bit more acceleration from the solar wind that streams out there.
Once you get out of the heliopause, the Galactic wind would be even stronger, which seems logical considering how much the sun's magnetic field is distorted.
It might take a while to get a fair percentage of C, but with a constant, albeit small acceleration, a flight time of way less than a century might be possible.
I've tried searching for data on the Galactic wind, but I haven't been able to find anything.
Since bother Voyager probes have left the heliopause (how do you spell it?), I was hoping there would be some data available.