C172 Heavy
Falcon Capt gave you some great tips.
Additional considerations:
--No matter how much a passenger may want open the urn and disperse the ash themselves, don't let them! Believe me, they will probably screw up and then you will have ash floating around the cabin. Even a minor amount is enough to give you a nice gritty feel between your teeth. It's not pleasant.
--This may go without saying, but on a 172, you will need to prep the window before flight by disconnecting the little arm that holds the window slightly open. You want the window to be able to fold completely open so you can get the urn as far back and away from you as possible. Once fully open, the slipstream will hold it there. All you need for this is a Leatherman or the screwdriver on your fuel tester.
--Whos airplane will you be using for this? You WILL have a bit of a gray streak along the side of the aircraft. If you slip you may avoid most of it, but there's always a bit of residue left on the plane that will need to be cleaned off...
I have done these at the coast and over Mt. St. Helens. In my opinion, if you are doing it for the flight time, it's really not worth it.....