I used a 4.5 inch hold saw for the baffle with Vans's repair kit. worked great.
Don't remember the size but if you buy an adjustable hole saw like this one (https://www.amazon.com/General-Tools-55-Cutter-Adjustable/dp/B00004T7P1/ref=sr_1_3/146-8676445-4029722?ie=UTF8&qid=1516765707&sr=8-3&keywords=adjustable+hole+saw)
you will have whatever size you need.
Using one of these circle cutter tools on the rear baffle of a tank with a hand held drill would be rather sporty.
The circle cutters work well when the work can be clamped on a drill press table.
Exactly what I did. Removed the tank from the plane. Secured it to the drill press and drilled the holes. I will not say it was "Easy Peazy" but it was uneventful.Using one of these circle cutter tools on the rear baffle of a tank with a hand held drill would be rather sporty.
The circle cutters work well when the work can be clamped on a drill press table.