Half a tank of 91 and half a tank of 95 will give you 93
I'm not sure if octane booster would do the trick....never used it...but that'd be the easiest/cheapest way if it'd work. Put a $4 can of octane booster in the tank and fill 1/4-1/2 tank and run it at the track like that.
I would recommend aiming a little higher like for 95 octane with a 93 tune. When I run pump 93 gas with my 93 tune it still has to pull timing.