I've never done what I describe below, but I have dropped ashes twice from my Warrior (PA-28-161) out the small storm window in a small rolled up biodegradable brown paper lunch bag. Not sure if it opened, but the ashes got where intended and I didn't bring home any on or in the plane. Next time, I'll try the technique below.
Place ashes in a small brown paper lunch bag. Not too full. You may need more than one bag. Roll the bag into a tube. Cut a length of cotton twine 1-1/2 times the length of your plane. Tie the twine's end around the tube's center and secure with a loose shoelace knot. Wrap the remaining length of twine neatly around the bag. You do not want to create a solid log, so wrap a bit loosely, in case it hits your stabilator/elevator.
Secure the far end of the twine to something inside the plane (not your body!). In the air, slow as much as comfortable (here's a chance to practice your slow flight), bank as steeply as comfortable (steep turns), and stuff the bag through the storm window. When all the twine has unrolled, the shoelace knot is slipped, and either the rolled bag falls intact or it unfurls releasing a cloud of ash. Retrieve the twine.
Let us know how it works.