I found Ed Guthrie's method (described below) to be very effective when I scattered my father's ashes from my Citabria a few years ago. No ashes came back into the airplane. It helped to make a few trial runs with flour first. I took a safety pilot with me.
This was done over our farm with the family watching down below. I came back around after scattering his ashes and did a victory roll and a loop in his honor (my father was a pilot). Then a friend flew up in his Extra and did an aerobatic tribute to him for the family. I made a video (put to music) of the event and gave copies to all the family members. Afterwards several family members told me that it was actually an uplifting experience and said that's how they wanted to go too.
Ed's method:
"I used a 3'x3' piece of bed sheet and a small (~8') length of kit string. Spread the sheet on a table top. Pour the ashes into the center of the sheet. Pick up the corners & edges making a sack around the ashes with a "neck" in your hands. Starting at one end of the kite string and working from the bottom of the neck working upward (start near the ashes in the "sack" part of the sheet) wind the string around the "neck", winding the successive loops over the free, starting end of the cord string. When 1/2 way up the "neck", fold the "neck" back on itself and wind around the folded "neck" back towards the "sack". At the end, pull a loop of the string back through the last wind and snug tight. At this point you should have a quick release knot around the "neck", not unlike a 1/2 of a shoelace knot, that will release if someone pulls the free end of the kite string. The free end should be ~4' long or more--sufficient so that someone in the cockpit could hold on to it while you push the sack out a window. Go flying.
In flight, hand a rear seat passenger the free end of the kite string and ask them to hold onto it. Slow the aircraft and open the side vent window (in a C150/152/172/182 etc. just open the pilot side window). Push the "sack" out the vent window, holding the neck in your right hand. When the "sack" is fully outside, extend your right hand out the widow into the slip stream, holding the sack by the "neck" just behind the vent window opening. Let go.
The bag will fly backwards until the kite string comes up tight, at which point the loop knot will release, the loops around the "neck" will unwind, the sheet will release from the kite string, and the ashes will disperse in a "puff" behind the wing. No ashes in the cockpit and no betting on which way the airflows through the vent window.
Pull the kite string back into the cockpit, close the storm window, and fly home."