jack
here's a picture of my charger drain-source voltage before I minimized the parasitic inductance.
The ringing is much more severe than your results - it's as if you have more damping.
Nevertheless, if you haven't yet shortened the length of wire between cap/igbt/diode, you still have sections of wire where the current is going from output current to zero with every cycle. Large di/dt and any amount of inductance will create voltage spikes.
I recall advice from the members of the evtech list: Minimize the length (and thus inductance) of any wire/bus/trace that has large changes in current (large di/dt). Minimize the area (and thus capacitance) of any part of the circuit that has large changes in voltage (dv/dt).
When I changed the setup, the waveform looked like this:
Actually, that looks more like your output. In these photos, I was using fast turn-on/off times with a low value gate resistor. I think Paul's is set up to have longer turnoff times. Might be another thing to think about.