I would try the thin 20bore "B" wads first. 2 over the powder, one over the shot. We have found this type of load shoots quite well in a cylinder bored gun.
In one with quite rough bores, I used 3 thin cards between powder and shot.
The reason for the thin wads, is they will not punch through the shot cloud to create do-nut patterns like heavier donnaconna wads are prone to do.(especially lubed).
I have been able to fire 10 rounds, one after the other, just using the thin "B" wads as noted above.
If those wads are too difficult to used, then making a .605" arch punch is another option.